[dnanian]

Hi, Dan. Yes, you can use the same drive to back up both Macs to the same drive.

Since you have two different architecture Macs (Intel and PPC), you'll want to partition the drive into two, using the Apple Partition Map scheme, which is startup-compatible with both the Intel and Power PC macs. Then, simply connect the drive to the Mac you want to back up, and direct the backup to the proper volume (partition)... and you're all set.

Hope that helps.