one time machine for two computers

You can partition it to do that.

But spreading your backups among different drives limits the risk. If you have two backups on one drive, an operating system error on one partition (or dropping, or crushing) can destroy both backups.

My humble opinion.

It's not really a very good strategy but if the drive is big enough and you create two partitions you can do it. Just make sure each partition is named appropreiately and setup time machine on each mac to point to it's backup partition.

Now you can move it daily from mac to mac and when time machine see's the volume is present, it will back up. When the volume is not present, it may or may not make snapshots depending on what kind of mac it is. You can read this apple article to learn more about snapshots

http://support.apple.com/kb/HT4878?viewlocale=en_US&locale=en_US

Please refer to this extract from the Pondini web site:

http://pondini.org/TM/4.html

Ciao.

I have been doing it for years as my Time Capsules are networked attached storage. To date, I've never had a problem.