[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.