Yes, you are all aware of which install is which and can point mod installers to the appropriate directory. Not everyone can. I will describe 2 types of multiple installs - one simply a copy of the game folder (good way) - the other - a second or third install from CD to another location (bad kind - only one registry setting for location is avaialable not three or four).
Yes I'm aware that experienced players probably know how to take a snapshot of their OP install and save it elsewhere, and can thus run multiple installs on one install location by swapping out folders (only installed from CD once). This will work.
Install from CD multiple times and which one gets the registry setting? The first or the last? When mod installers look for the install directory they look in the registry (unless you all want installers that search your entire hard-drive for OP installations and prompt you for which one to install to... could increase install time by up to 20 minutes or more on some systems...). In this case you must carefully select which copy the mod will install to).
Additionallly things are more complicated than two years ago... people are running multiple OP+ installs too (probably the source of people wanting multiple installs so they can stay on OP+3.4) the thing is OP+ uses only one registry location for its vversion information and install location also... so you see multiple OP+ installs complicates things further...
If you know the game inside and out and run multiple installs, theis thread is not for you, it is for those not so handy with the computer or explorer yet still insist on running mulltiple OP installs.
(all I have time for right now - I'll add detailed instructions, contingincies and complications of multiple installs)
...and don't look at me like I don't know what I'm talking about.