When applying to university in the UK do you have apply for accommodation at both of your chosen universities or just first choice?

You apply for accommodation separately at each university you apply to in the UK. Your application to the university itself is distinct from your application for accommodation. Even if you only intend to go to your firm choice (first choice), you should still apply for accommodation there *and* at your insurance choice (second choice) in case you are not offered a place at your firm choice and end up going to your insurance choice instead.