Iomega hard drive not recognized on MBP Retina when another computer is connect

Hello!

I've got an older Iomega Ego hard drive, using it with Time Machine to back up two macs, a desktop tower MacPro with OSX 10.5 and a new MacBook Pro Retina with 10.7. With both computers I'm connecting with FireWire, as there are two firewire ports on the hard drive.

If I just plug the Iomega into either ONE of the computers, there is no problem, and the computer can recognize the hard drive and can back up with Time Machine. However, if I then connect the second fire wire cable to the second computer (doesn't matter which order), then the second computer does not see the hard drive, and instead shows it being connected to the first computer under "Shared." This is still the case even when the hard drive is ejected but still physically plugged in.

Ideally I'd like to be able to have both computers plugged in and backing up with Time Machine regularly - maybe it's not possible to do it simultaneously?
On the hard drive, it shows a folder for each computer in the Backups.backupdp folder.

Any ideas? Thanks!

It's not possible to do them both at the same time. And, actually it might be best to buy a second hard drive to use with your new Retina. That will avoid the extra wear and tear on the iOmega. Also, you can always install another hard drive in a spare bay in your Mac Pro to use as a TM backup.