If you want both hooked up but only while using one, The Lacie drives usually have multiple ports to hookup the computer. Use the FW800 on one computer and the FW400 on the other (or USB)
The easiest way to do this would be to set up a wireless storage network. The best way to do that I know of is to buy an AirPort Extreme base station, which has that capability. This way, you can share multiple disk drives wirelessly in write mode with multiple Macs.
A cheaper alternative is to connect one of your MBPs to your external disk drive and enable fire sharing on it (via the sharing preference panel). On the MBP that has the disk drive, select its icon and get ino on it, then make sure the permissions are set up properly on it. You can then mount the disk drive on the other MBP.