Time machine, at least as of 10.5.6, will gladly maintain multiple Time Machine disks. This way, you can periodically rotate your Time Machine backup disks to keep one safely off line and off site. The trick is that there is no trick. I say this because there are some dangerous methods posted on the internet involving complex setups like copying the hidden disk ID number, that while appearing to work, actually slowly corrupt your backups.
All you need to do is attach the disk and tell Time Machine to use it. When you are ready to swap drives, attach the other and tell Time Machine to use it instead. The very first time you do this, there is (of course) a big time consuming backup. After that, the swap is pretty fast. A bit longer than a typical backup session (but a lot shorter than say an rsync by comparison).
Time Machine correctly finds the files that were changed since the last time that disk was mounted and backs these up, making a perfectly synced new backup in the older disk's time lineage. And then it proceedes normally. If you are alternating disks weekly, then of course each disk is missing the disk state from the previous week. It still has all the files, it just can't restore to an epoch during which it was not connected -- that time period state is on the other disk.
Other than that, this approach seems to be significantly better than the alternative of keeping an rsynced copy of the Time Machine disk. That's an ugly process, due to all the hard links, that is fraught with many small perils.

