I always think its best to split, very few circumstances to not, assuming both have at least gold.

You will each get points (can be combined later if that matters), you each get the night credits, and better chances of upgrades. In addition, if they are running a promo where its an extra number of points per stay, you both get those. My wife and I almost always book 2 rooms, since we travel with 4 kids. We have never not been giving neighboring rooms, and both rooms get all the benefits.