TM backup of external drive fails - Apple support says not possible

Hello. Trying to use time machine to back up my external hard drive. Here's the setup: Mac Mini, External HD #1 500G, External HD #2 1TB, both connected via firewire.

1) Ideally I'd like time machine to back up the internal drive AND external #1 to external #2 (and ultimately mirror #2 to offline, e.g. if Carbonite ever starts supporting external drives).

2) I have tried to set up the options in TM to do this (i.e. removing exclusion of external drive #1). TM churns and seems to be preparing but it fails with an error message (can't copy to backup drive).

3) I spoke with apple tech support and they say it's not possible, which seems odd in itself and different than what I've read here.

Questions:

A) Is it possible? If so, can someone suggest best approach to troubleshoot? (I've already checked permissions and drives. Also excluded folders that looked like they might lead to recursion errors, though I would not have thought that would be an issue.)

B) If not possible, is there a solution to automate backing up both the internal and external #1 to external #2? I looked at SuperDuper and it seems to allow for one source and one target only. I tried to set up Intego Backup Assistance (which allows multiple sources) but it fails with "unexpected error" message. (I seem to be able to manually backup selected folders though.)

Without an automated solution the best I can think of is backing up #1 to #2 and backing up internal to Carbonite. I don't like splitting up my backups though and besides, the majority of the data I care about (photos, movies, music) are on external #1.

I'd be very surprised if what I want to do is an unusual situation. Has anyone got a similar set up working?

Thanks!