Can I back up two different macs to the same disk using time machine?

I don't know if you can, but I would not. It would only create confusion if you needed to do a restore. Instead, I recommend formatting external the disk into two separate partitions. Name them something like Backup 1 and Backup 2. Then direct each Time Machine to its own partition. Be sure that you get a large enough backup disk. Each partition should be two or three times as large as the HD it is backing up.

You are most welcome.