Technically they can. What is actually prohibited is for the electors of a state to cast both President and Vice Presidential votes for candidates in their own state. (Constitution, Article II Section 1). So two candidates from New York, say, could receive votes from electors in all of the other states but from New York electors they could only receive one. In a very close election that might make a difference, but for the most part, it'd be fine (unless you happen to be a New York elector).
I listened to Trump’s interview with Pat Robertson and one interesting clue came out as to who would be his VP. Trump said it would have to be someone with extensive political experience who knows the ins and outs of Washington and who would know how to get legislation through the Congress.
That was quite interesting.
Here is the link:
http://freerepublic.com/focus/f-news/3401312/posts