I just like the names to be meaningful to the parents, and not too similar. For instance, I know triplets named
Angelica,
Angelina and
Angela. That is horrid in my opinion.
If I had twins, I'd name them using the same letter. That way they are linked, but not too matchy. I just happen to like many names with the same first letters. I don't think you should go out of your way to make them sound matchy.
I'd go with these:
B/B:
Gavin &
Gabriel or
Dean &
DamianB/G:
Damian &
DianaG/G:
Frances &
Fiona (although, there are so few girls names I love that I might name girl/girl twins with different letters, perhaps some combination of
Serena,
Diana, Ellia &
Fiona)