I'd go for 3hrs, just because the queues at immigration on arrival can be quite long (I arrived early morning, around 7am), though it can be done quicker. I had about 4 hours in both directions and had time for breakfast landside (within the terminal) and in one direction had to send some urgent documents for which I had to switch to the main KLIA terminal to go to the Malaysia Post office (spoiler, the documents never arrived). 2hrs is tight but possible, but just go for 3 and don't worry about it, especially as they won't sell it on one ticket (same in my situation, so essentially 2 separate return tix which I nested)