Mac – Lacie rikiki (500GB) Time Machine

Yes - there are two ways to do this.

Apple intends and designed Time Machine to work with disks in a sequential manner. You are expected to fill up the first disk and put it on the shelf. Then attach the second and fill it up. You can then go back to the first and this lets you have more history before the oldest backup is reclaimed due to space running out.

An alternative is to use Mac OS X built in RAID functionality. You will have to erase all the disks when you put them into a RAID the first time - but you can have two disks with half the data written to each every time a file gets written. This is striping or RAID 0.

Using a RAID would mean both drives need to be connected all the time. You would then have one large backup and since Time Machine won't be saving two copies of many files - you will get much longer time for storage using a RAID than the first method of using the drives sequentially. If your boot drive is almost full - I've seen times where the first method could store a month or two of history, but years would fit on the second method. It really depends what files you have and how often they change.

TM will not move your old snapshots to a new drive, although I'm pretty sure that if you first used Disk Utility's Restore tab to clone your old backup drive to the new one (after reformatting it properly), it would see the old snapshots properly.

But there's a bigger problem. You are trying to use TM as an archiving tool, but that's really not what it's designed for. TM will expire old snapshots as it sees fit, so if you want reliable retention of old files it's not the right tool for the job. Unfortunately, I don't have a good suggestion for a better tool, but I would strongly recommend looking for something more appropriate.

That said, switching TM to a new drive (and removing the old one from its backup target list) would be a semi-reasonable way to retain all of the old snapshots (and having multiple redundant backups is a good idea). Doing that and switching to a new archiving tool would also be reasonable (since you probably won't be able to migrate the old TM snapshots to any net tool you select).