For clarification I'm the poor soul that Yerserf games with
I have activated multiple at once but so far I haven't come across any problems with doing so.
I have heard that, yes this can cause problems, but I normally don't use more than two at a time.
Here is what happens personality wise (I did edit the audio so no physical changes occurred, just the mental state changes)
Rainbow Dash...Little harsh, need to keep going, nothing but being first is an option. Fun to use when I'm in game.
Apple Jack ........My work ethic skyrockets and I need to do something physical otherwise I feel touchy and ready to break something
Fluttershy...........In public i shrink into a ball but alone its nice due to me just wanting some quiet time. its also easier to draw
Twilght Sparkle..If there is a problem to solve or something that has to be done it will be done, there is a down side as I won't be able to do anything else till the current task is completed
Rarity.................Everything must be clean. it must also look good. best way to describe this is I got on to Warframe (awesome game BTW) and spent 3-4 hours redoing all of the color options on every weapon and warframe. this one scares me a bit
Pinkie Pie...........OH MY CELESTIA. I have only used this once and I'm really not sure if I want to ever again. I went totally scatterbrained and couldn't focus on anything for more than 2 seconds. the reason I say its bad because I couldn't focus enough to turn it off. Till i decided to go and make some food, then i could focus on the silly word
That's my experience and to be honest, its fun, but if you are going to do this, read up on it, know what you're doing and keep the erase file handy just in-case.
P.S. try playing your favorite game with each personality its quite enjoyable :comeatus: