can i backup 2 macbook pros to a single external drive?

You should be able to do it by partitioning the drive?

Each backup will use its own partition. There should be a lnk at the top of ths page.

Make the backup ideally twice the size of the internal HD. If your two Macbooks internals total 1TB get a 2TB external drive.

Yes, but the computers will need different names. There is no need to partition.

I would go with 2-4 times the hard drive sizes.