Used 2 egg yolks instead of 2 whole eggs in a cheesecake - how bad is it?

Eggs work in a cheesecake mixture to help hold things together and set up, much as they do in custards and puddings. The yolks bring fat to the table, the whites bring protein - so together they will help the cheesecake become smoother and firm up. Yours may end up a little less firm, and perhaps a little less smooth- and it might be a bit denser, but I expect it will still be quite tasty. I'd go ahead and chill it overnight, and check it after it's had time to fully cool and set up. Even if it does fail to hold a wedge shape, serve it up in a bowl with a nice topping- cherry or other fruit perhaps, and call it a deconstructed cheesecake dessert. Nobody needs to be any wiser.