Rotating objects in opposite directions

How can I rotate two different objects in opposite directions?

Currently, I rotate one object one way over 30 frames, and the I switch to the other object, and turn it the other way, even if I have the increments locked at 90. I’m rotating the objects 180 degrees, but I don’t want to use a premade animation, I want to do it myself. (In case I needed to do it in such a way that the pre-built animations couldn’t handle it.)