It's not just the credit card promotions, I've got hundreds of thousands of points and miles from promotions and not a single one of them from credit cards.

When Hilton runs it's stay 4 times and get 50,000 bouns points, you can do it twice with two accounts, not once if it was a joint account. When the airlines offer bonus points for using the kiosks to check in, again, you get twice as many miles.

If you are only earning points through the credit cards, then it won't matter. Two people spending $500 equals the same amount of points as one person spending $1,000.

But if you are both taking Amtrak, then two accounts will get you points for both of you, things like that.