Get an OWC Mercury Elite AL or a Cool Drives, something that comes with an Oxford 924 chipset, which will give you 2 FW800, 1 FW400 and a USB2.0 port. Then you can run FW800 to each machine. I would recommend getting either the Dual Case and a couple drives, or a prefilled RAID solution, OWC has them in 800GB(2x400) and 1TB(2x500) models on sale for $315-350. You can then set them in JBOD mode so that each drive is independant, using one for TM and the other for Shared Data. You may even want to partition the TM drive so that each machine has its own part, though honestly I would use one of the two drives for shared, the other for TM on the laptop, and add another internal to the bays for TM on the MacPro (assuming that you still have 1 of 4 bays open, and that even though "internal" TM will let you use an internal non-system drive on a MacPro for backup - the real fear is that your main fails, not that the whole system burns up and kills all you internal drives. It seems a real shame if Apple has a tower that can hold 4 drives but would force you to connect an external for TM).
Anyway, yes, get a Dual case and drop a couple drives in there.
800GB
1TB
Case
Cool Drives