i don't know if it is possible or not as i don't really understand the underlying technology of messenger, however i just had a cool idea for a script, i keep encountering the problem below from time to time.

"if in messenger on my main PC and i then boot up my other pc it signs in my messenger account on the second PC which in turn kicks me out of my account on the main PC forcing me to have to sign back in on main PC."

can a script be built so if my main PC is logged into WLM it won't let me login at an alternate location without first either turning said script off or signing out of the Main PC first?

I think that is impossible...
you can't prevent an external messenger from logging in...
the only thing you might be able to do is automaticly login again at your main pc...

so theres not a way to send an error message to the second messenger saying something like "login denied as an instance is already in use" so it can't complete login, or create a script that runs on the second PC that instructs the second PC not to login at all unless a password thats stored in said scripts settings set by the user is entered manually preventing the automatic login at second location.

Why not just disable the "Automatically run when I log into messenger" option on your second PC.  Then it won't start on startup but it will still sign you in automatically when you're ready to use WLM on that computer.

yeah i suppose that would work too however it doesnt always work in all versions of messenger atleast thats been my experience.