When I did mine I had the bike on a rear paddock stand and took the left cap off first.

Sure enough I couldn't compress the spring enough with the spacer to allow me to get the cap back on. However, all it took was someone to lift the front end of the bike enough to allow me to get the cap on. Having a spacer in the left fork leg made the right fork much easier to do - I needed no assistance from there.

To make the job easy, just prepare the bike by lifting the front end off the ground. By making the job easy you are less likely to strip the fork cap threads too.