So if you follow the school of thought that says having repeating phonems sound between the first and last name sounds pleasant than you'd want to find a first name that has some phonems in common with James and some in common with Milburn.  One way is to find a name with an internal "m" sound because those should work best with either name.   I typed your names into the matchmaker but didn't see any names from their suggestions that had a "m" sound.  Maybe your own list does though?  So then I started looking for a name with a sound from both of the last names.  I soon realized that lots of names sound lovely with James.  So I'd focus on finding names that work with Milburn first and then see how they suit James.  I think names with an "L" or "N" sound  might work.  I think the "L" sound in particular softens Milburn some. 

Some examples of what I mean:

Leah has been on my mind lately.  Leah Milburn?  Leah James?

Or on the boy side Eli Milburn or Eli James; or Nathaniel Milburn and Nathaniel James. 

I hope this is making sense.  It is a hard challenge because you also want to avoid first names ending in "m".  Like Liam first struck me as a good choice but the "m" would slur together.  Also and some names ending in "s" or "n" may not work (although some might be fine).  And a name like Caleb seems to emphasize the "b"s in Milburn in a less than musical way.

Good luck.

I agree Dylan Milburn doesn't flow as nicely as Dylan James & it's too bad about the initials.  But I don't see a problem with Jonah Milburn at all.  I actually like it better than Jonah James as I'm not usually a fan of alliteration.

Emily Milburn is lovely.  Yes, the "mil" does repeat-but not in an overly rhymey way and Emily is such a classic name that it seems to "fit" with most surnames.  Aria Milburn is also fine, but I like Emily a little better with Milburn.

I say take Dylan off the list because of the initials, but I don't see any reason why the other names won't work.  

I like another Laura's suggestions for you, but can't help but wonder why you don't use James as a middle or second middle? Since is is accepted as a fn and surname for boys. Dylan James M works quite well and avoids the bad initials thing. Even Dylan Aaron James M works though it is a long name.

As far as the girls, I think it works okay with Emily Sophia James M but Aria Sophia James M is also a bit of a mouthful. Other names I thought of:

Dylan Robert; Dylan Henry; Lucas Wesley; Benjamin Isaac; Evan Patrick/Cooper/Carter

Ainsley Renee;  Leslie Anne; Hannah Sophia; Samantha Katherine

I think these names work with either last name or a double middle/last name as I mentioned. These names may not be at all to your liking though.

I don't normally make name suggestions, but the name that immediately came to mind was Adrian, so I figured that it couldn't hurt to share. Adrian Milburn and Adrian James both sound good to me. There is a sound repeated in both surnames (though it is a different sound in the two cases,) and if you aren't adverse to alliteration, you could even do Adrian Jonah Milburn/Adrian Jonah James, if Jonah is a name that you really like.

I also second Lucas as a middle name: Adrian Lucas Milburn/Adrian Lucas James.

Anyway, just thought I'd throw that out there.

I agree that Dylan Milburn is a mumbly mouthful, but Jonah Aaron Milburn is very nice.  So is Jonah Aaron James.

Same thing for the girl names - I'm not a fan of Emily Milburn, because it repeats the m-i-l sequence. But Aria Sophia Milburn and Aria Sophia James are both great.

thanks everyone. goodness its so confusing!

You are all thinking along our wave length as our first is Natan Alexander James Milburn, with James as a middle name!  ...and if we get married we plan to use James as a surname and drop Milburn, the registar said that would be fine and reasonable.

Adrian is a family member so is a no go. Does Aiden work?

I love this brainstorming thank you so much for your help, I am quite sold on Dylan Aaron James Milburn and Jonah Aaron James Milburn (though I prefer Dylan mainly because of the meaning) but I like having a couple of choices which can be thought about once we meet the baby.

still stuck on the girls names mind. Its the swapping the surnames thing and the fact that James is a really common name vs a more uncommon one thats getting me as I discussed above I think.

You are right though so many names go with James.

What is the time frame for the Milburn-to-James switch? If it's matter of months or even 2-3 years then I'd just focus on what sounds well with James. (Unless it's absolutely horrible with Milburn). I'm not seeing any issues with Dylan Aaron Milburn (it's not DAMN, just DAM) nor with the other names on your list - I think they're fine with both Milburn and James.

Another thought: Have you considered using James as the "social" surname? As in, the name on the birth certificate is Milburn but you introduce the child as First Middle James to everybody else. The official switch to James would then just be a matter of paperwork.  

I'd focus on naming off Milburn. James is one of those last names that just about anything sounds good with it (I call this the Jones effect, and James works similarly to Jones).

Jonah works pretty well. How about John?

Other names that might work: Devon(either), Ezra(b), Iris(g), Connor(b), Lyra(g)... The standard two-syllable, accent on the first syllable, balances well with Milburn's cadence, and doesn't interfere with James.

An M- name might help with the flow. The alliteration helps with Milburn, and doesn't interfere with James. Madison, Mark, Madeleine, Matthew, Michael... You could even take advantage of the alliteration and use a rare choice: Montrose, Madrigal, Myla, Marchand...

My vote would be for Jonah Aaron M. or  Jonah Aaron James M.  Dylan Aaron is fine if that's your favorite, but going on flow alone I don't think they go together nearly as well as Jonah Aaron, due to having the same syllables, stress, and -n ending.

On the girl side, I prefer Emily.  And I love the suggestion of Iris, if you would consider that.  The sounds are not toooo far off from Aria, and I think Iris Milburn sounds so classic and wonderful. 

I like all the suggestions for adding James in for a middle name.  Maybe once you do the switch you would want to keep Milburn as a middle name for your son?  Dylan Aaron Milburn James?  As a reminder of his parents unconventional love story.

I'm not a huge fan of Emily Milburn because of the 'mil' repetition and I think Emily James might run into a few more Emily Jameses in her lifetime.  I prefer Aria James, though not with middle name Sophia because it makes it sing-songy.  Aria Milburn is a bit choppy because of the strong r in Aria. 

Some suggestions that have that pretty -y ending in Emily but no -mil syllable, and at least 3 syllables:

Cecily (or Cecelia)
Elodie
Avery
Natalie

and some alternatives to Aria without that harsh -r sound:

Talia
Julia
Nadia
Olivia
Lydia

So the child's surname will switch to James at some point? If that's the case I wouldn't worry about matching with Milburn, unless there is a chance you will keep the Milburn last name. I think all the names you have sound fine with both James and Milburn. For the DAM initial issue, you could go with Dylan Jonah Milburn or Dylan Aaron James Milburn instead.