Neither. Or both. Chances are that the 1600 will run dual channel at 1600, and the 1866 (that's usually an XMP setting anyways, default is 1600) will run single channel at 1600. That's assuming it'll work. There's absolutely no way to tell or guarantee it will. It's pot luck when mixing ram sticks from different kits. You can have the exact same ram, from the exact same vendor, from 2 different packages, and only 1 works at a time.
It won't hurt to try, it might help if it does, but there's a good possibility it won't.