When installing M&B (not patching but installing a new... install) you get to choose in where (which directory, that is) to install the game. Normally this is C:\Program Files\Mount&Blade.
Now, lets assume you have .731 installed in C:\Program Files\Mount&Blade and want to have an installation of .751 in a different directory. Just download the installer, then when it wants to install to the directory you have the previous version in, change that directory so that you install to C:\Program Files\Mount&Blade751 instead.
But remember that every install creates it's shortcut on the desktop and thus if you want to have several, you must create the others manually.