As sibling comment notes, you can switch between WMs at runtime, you just have to not exec the wm from xinitrc. Instead, you run a terminal emulator or whatever as the first X client, and then from there run the window manager like a normal program. I mean, I'm sure there are nicer ways to do it, but that worked for me.

Alternatively, if you want to run multiple at once, you can run Xephyr, which nests X sessions, and then you just run whatever clients you want in each child X server, including the window manager.