NO, unless you can find a method to have both Mountain Lion and El Capitan booted at the same time?
I didn't think so!
Fast switching is for switching between two (or more) users of the same boot system. If you had 10 different user accounts on your Mac, you could have all ten available for fast user switching. ALL would be using their own part of your RAM, so you would want to make sure that you have max memory installed (4GB would be REALLY draggy on such use, for example).
But, all users would also still be using the same system, which is the one that you are booted to.
The only way that you could do what you ask, would be to have one of the boot systems in a virtual environment application, such as Parallels, where you are booted to one system, say El Capitan, and running the other system, Mountain Lion, in a virtual machine (VM) under Parallels or another virtual environment app. In effect, booted to El Capitan, with Mountain Lion running as an app - like you can do with Windows under Parallels.
I know this can be done with Snow Leopard. You have to be using Snow Leopard Server, as that is the only way that is supported in Parallels.
I don't think it is as easy with later OS X versions, as the Server is an add-on to the system, and not a separate OS X version as it was with Snow Leopard and older OS X versions.
If you really want to pursue it, you could post a question about that at the Parallels support forum. Someone will probably respond and tell you "it doesn't work", or "Here's how you can do it"
I don't know what response you will get, however.