I have a Kinesis Advantage with browns switches, and I got the sampler pack of O-rings from WASD, and tried a lot of different combinations. You can definitely put 2 O-rings on the same key and it reduces the travel.
Here's my memory of things I've tried:
- Black O-rings give you a hard landing. I actually thought that was what I wanted at first, but after typing on them for a while they kinda hurt your fingers from bottoming out on that hard surface, so I switched to the blue ones, and then to the red ones.
- I wanted short-travel at first, and so i tried putting 2 black O-rings on each key, and it worked but again, I didn't end up liking it in practice.
- I actually even put 3 black o-rings on some keys, and it worked too, but it reduced the travel so much that sometimes the key wouldn't even actuate, depending on how hard I bottomed out.
- like @badugi, at first I was naively trying to recreate the feel of the apple laptop keyboard, with the short throw (~1mm!) and hard landing. But in practice, I think that's a waste of effort and my new clear switches feel so nice that I've mostly given up on that pursuit now. But if you want short throw, the best way to do it is Jailhouse Blues. I've tried that on a few keys on another board and like it, but haven't done a whole board to try the real day-long typing experience on it.
- the only exception to the above is that I left the double-black o-rings on a few of the thumb keys, like my "enter" key and I think maybe the space key too, because I tend to really pound down on that enter with my thumb and it feels better to have it have a hard landing than a squishy one.
- between the red and the blue o-ring, my experience is exactly how WASD advertises them -- The blue ones are a little bit more squishy and reduce the travel a little more, and the reds are a little bit less squishy and reduce the travel a little less. Both feel nice, much nicer than the stock switch/cap when you bottom out.
- I ended up using blue o-rings on most keys, and red ones on some of the pinky keys so they have a little longer travel. not sure why but it felt better to me for the pinky keys.