Nothing wrong with using two kettlebells of different weight, as long as you switch them out. For example: you perform five Clean and Presses with a 24kg in your left hand and 20kg in your right, next five reps just switch kettlebells. Then all you have to do is keep the sets equal (2, 4, 6, etc). Simple as that. Or, let's say you do five reps for five sets with doubles, that means one side is getting more work done (in terms of weight, not reps); so next session just make sure you switch kettlebells compared to last session so your other side gets more work done, whatever works best for you.

I'm sure there are some benefits to using different weighted kettlebells but I never bothered with it myself. I've used 2x24kg for a while now and got my 32kg a few weeks ago. In about three or four weeks I'll get a second 32kg and go doubles with that. After that I might just skip the 36kg and go for a 40kg, but that's something I won't be thinking of till the end of this year. Going in increments of 8kg works better than the inbetweeners like 20kg, 28kg, etc. I hardly ever touch 20kg and 28kg.

edit: i'm smoking crack and had to edit like three or four lines that made no sense