I took a stab at the RH one with the engine in the car - the gasket ended up folding up inside the cover upon reinstallation, and as you can imagine, made a hell of a mess all over the floor.
Since my clutch was making a little bit of noise anyway and I feared having to install a TSK3, I just yanked the engine out and did both rocker cover gaskets at the same time. Hell of a lot easier with the engine out of the car. (Though I've been told you can raise the engine up a few inches to give the space you need to do the covers.)
But since it took me the same amount of time to do the passenger side cover gasket with the engine in the car as the process of pulling the engine and reinstalling it, I'm not sure which route I'd take if I had to do it again. Would depend on how much space I had, I suppose.