The advantage is that you're basically collecting more light in the same amount of time as imaging with one scope, so you theoretically end up with a better signal-to-noise ratio than with one scope.
For example, in Nick Risinger's sky survey he imaged with
six Zeiss 85 mm lenses + FLI ML-8300 cameras on one mount. With only one camera/lens, it would have taken at least six times longer...
http://skysurvey.org/survey/
As for different focal lengths, you might combine data from two different scopes to as a way of getting both a wide field and depth in the target of interest, as with Mike & Rolf's recent collaboration:
http://www.iceinspace.com.au/forum/s...d.php?t=108211
So why don't more people do it? Lots of reasons that I can think of:
1. getting one scope with one camera imaging "perfectly" is hard enough on its own...
2. $$$ - two scopes, two cameras, two focusers, two guiders, two sets of filters, much beefier mount, etc.
3. instead of multiple smaller scopes with small chipped cameras, you'll (probably) get a better result with a single high-end scope/camera - i.e. lots of data from a blurry scope won't give you as good result as half the data from a high quality, well-corrected scope
4. more equipment = more things to go wrong during the night