If you can get a tailcap with switch, you should be able to install it in a sideswitch body -- then you'd have to have both switches on, so if you leave the side switch on, then the tailcap switch controls it, and vice versa. I think I've seen where someone did that, but not certain...
I wouldn't do that with a light I might need suddenly, of course -- if both switches are off, you have to click them both to get it on, and you'll probably have to click one of them two or three times if you don't know their states.