I think it's a bit silly, but fun. The only ones I can think of right now are
Eliot and
Elliott, and
Alan and
Allen.
They could also go with matching nicknames for different names, e.g.:
Ben - but one of them's
Bennett and the other is
BenjaminCal - but one's
Calvin or
Callum and the other's
CalebChris - but one's
Christopher and the other's
Christian, etc etc etc.