I have been using 2 instances of FARR running simultaneously for a bit now  -- below are some of my observations, followed by a sketch of what I did to set things up. May be there are some others who are doing the same and wouldn't mind sharing their experiences.Some advantages
- Can test out new versions of FARR more conveniently -- just upgrade one of them
- I am not FARR-less when one of them crashes
- Don't feel nearly as disabled when testing plugins in one of the installations and things aren't quite working -- I have a "left" FARR (appears on the left side of the screen and is invoked via double-left-control ) which is sort of my "stable" configuration and a "right" FARR (you can probably guess how this one works) which is more of my "development" configuration. I tend to test with the right-hand one -- it ends up crashing (perhaps mostly MS' debugger-related) more often and consequently needs to be restarted more frequently.
- Can use KlipKeeper to interact w/ some of the FARR dialog boxen that I couldn't before.
Interesting and miscellaneous things
- Portions of configuration information are not obviously shareable -- you don't want everything the same because you may want to maintain different sets of window locations, hotkeys, skins, icons, and plugins (and perhaps other things). OTOH, you might basically want the same Search Folders.
- Another app is running all of the time -- but I've found FARR to be pretty light, so this hasn't really been a problem for my setup.
- Setup is a slight bother
- Upgrading and ongoing maintenance of configuration might be a pain depending on your situation -- it's been so-so for me so far
- Using Dcupp?dater and/or interacting with various FARR-related configuration dialog boxen may be a bit confusing -- AFAICT those things aren't affected by skin changes
- Can drag back things between FARR windows -- say from the results list of one, to the text field of the other.
- Another reason to have skins and custom icons
0. Quit any running instances of FARR
1. Copy an existing FARR installation directory and rename it
2. Rename executable of new installation to not conflict w/ any existing FARR executables (I add 'dev' or something else right before '.exe')
3. Start the new installation's executable
4. Customize the skin and tray icon (Options -> Settings -> Advanced visuals)
5. Change hotkeys to not conflict w/ any of the other installations (Options -> Settings -> Hotkeys)
6. Move FARR window to a location where any of the other ones won't appear
7. Tweak anything else desired
8. Restart other FARR instances
That's what I remember at the moment - I may have missed something.
 I've noticed a place or two where this has come up and thought I'd "factor" out a thread as I hadn't succeeded in locating a dedicated one.
 Thanks to Tap Tap Hotkey Extender.