Kara is Clark Kent's biological cousin.
Kara: We don't have time to wait
for ancient technology. Between the two of us, If we have to be frank to each other?
we can cover more ground. Of course, it would help
if you could defy gravity.
"Between the two of us" usually means "this is confidential, just to be shared by us two." Another folksy expression is "between you, me, and the bedpost," implying a secretive conversation in a bedroom (although people say this anywhere).
It's possible that she just means literally that the two of them can cover more ground than just one, but I can't tell without more context.
Thanks, D!