Romney, not so much. Romney may pick Santo just to get the Evangelical vote. I believe if Romney would have won the nomination in 2008, he would have picked Santo then, or else Huckabee.
Romney is in no way working with Santorum. Romney has promised the VP slot to Rand Paul in exchange for the support of Ron Paul in attacking the other conservative candidates. Notice, you haven’t heard anything from Ron Paul, or at least not for the past few months, attacking Romney.