Nothing in either of your Macs would automatically format a disk without informing you first and presenting you with the option of cancelling said action.
Yes, you can use a single disk to back up two Macs, given that they do not share a common computer name.
Be aware that if you have a multi-interface external drive (one with USB, eSATA, FW400 and FW800 or some combination therein) that the drive can only be hooked up to one computer at a time. You can't "get tricky with it" and connect the MBP to the USB port and the other computer to a FireWire port -- they don't work that way -- so you'll have to keep drive use independent and mutually exclusive between the computers.
A typical backup procedure would look something like this:
1) Connect Time Machine disk to computer 1 via USB/Firewire/eSATA.
2) Time Machine does its thing.
3) Properly unmount/eject the Time Machine disk from computer 1
4) Connect Time Machine disk to computer 2 via USB/Firewire/eSATA
5) Time Machine does its thing.
6) Properly unmount/eject the Time Machine disk from computer 2
7) Repeat until your wife stops you