[andrew1]

Nope, you can only apply to one college at either Oxford or Cambridge per year (you can't apply to both universities, which probably makes sense as almost everyone who applied to one would probably apply to the other). There is a clearing system I think where if the college you apply to doesn't want you then other colleges can look at your application and choose to interview you, but I think that's fairly rare.

[flipbrad]

Whilst you're discouraged at all levels from applying to both Ox and Cam, I don't think it's true that you can't. But you're correct about the second interviews (at other colleges). This was actually pretty standard (for Biochem, at Oxford) when I applied and you went for the alternative interview (at a college you're assigned to, you don't get a second choice) before the first has made any sort of decision on you. There is a second safety net, whereby you can be 'pooled' (which is basically what you describe); if you don't have a college preference from the outset, you can apply directly to the pool (again, this is Oxford, not necessarily Cambridge)

[ig1]

You can't, UCAS won't accept your application. There are a few exceptions where you are allowed to apply for both, but in general you can't.