I would pick names that would make each child distinct from the other. I would choose names that go together as a sibling set, but would not have them start with the same letter nor do I want the names to sound/ look similar. I would use a theme as long as it was not too obvious.
As of this moment I would name twins:
B/B: Hendrik & Johannes
B/G: Leif & Freja
G/G: Vilhelmine & Sybilla
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PNL:
http://www.behindthename.com/pnl/126941