Yep. You can either install mirc in another directory (you can use the mirc installer or simply copy and paste the mirc.exe [and mirc.hlp if you want]), installing twice may have some sideeffects on registry settings (not sure, but it may overwrite the earlier versions entry in the "add/remove programs" control panel) and shortcuts, none of these seem major though.
You can also use the -i<ini.file> command line parameter (just make a shortcut with the parameters) to have the same mirc run different configurations (scripts, nicks etc.) with multiple instances of your mirc.exe - which will save a little disk space and clutter. See /help command line for more information.
Hope that helps.