Cash is a very common gift around here. I'd say about half of the guests gave cash when my husband and I got married. I got lots of practice signing my new name, endorsing all those checks.
We tend to give actual gifts, though, not cash. Actual gifts seem more personal to me than just dashing off a check.
Anyway... I'd say, contribute to Mr. Pi Phi's cash gift, and have both your names signed on the card.