Apparently as an undergraduate you cannot apply to both universities. The reasons whay and ins and outs are well documented. However, I'm sure in my day you could. Does anyone know when (what year) this changed?
I couldn't tell you when it was changed, but I know my brother-in-law went to Oxford in the early 70s and turned down an offer at Cambridge to go there.
My memory of this is rather different - I think this 'rule' has existed at least as far back as the early 70s - and maybe much longer.
I think if you ask the question on this website, you stand a better chance of getting a qualified answer.
http://www.thestudentroom.co.uk/
My memory of this from the 70s is similar. Certain prestige Unis, not just Ox & Cam would not consider applications from those who hadn't put it as their first choice on the UCCA form (as it used to be called).