Personally I love patterns in names. So, when I look at you children my first thoughts are about matching syllables and connecting lengths. I know that may not make much sense so let me explain that.
[name_u]Riley[/name_u] = has 2 syllables and 5 letters
[name_u]August[/name_u] = has 2 syllables and 6 letters
[name_m]Bronson[/name_m] = has 2 syllables and 7 letters
In my mind you twins names should have 2 syllables and one have 8 letters and the other 9 letters.
Here are some examples that I think would go well with you boys.
Girls, 8 letters and 2 syllables:
Boys, 8 letters and 2 syllables:
Girls, 9 letters and 2 syllables:
Boys, 9 letters and 2 syllables:
If you wanted you could just to the consecutive letter numbers and not the syllables, because people pronounce names differently. I could pronounce a name and think it has two syllables and you could think it has 3. So it is up to you, this is just my personal thoughts. This way each of your kids will have their own initial.