While this is a good idea, but I myself disagree if this is the case.
Considering grinding MMO takes so much time, hence why you need to invest time doing it. If this happened to be implemented, I see that its already against what grinding MMO should be. You are no longer spending your time doing it.
Imagine me and my 9 alts afk for IG farm while maxing my taming, that would be very OP.
But I am just saying, with a little bit of "smart design", I am pretty sure you can grind few MMO together at one time. You just need to find the right one.
Just my 2 cents. No hatred :p
Edit: also you should feel lucky its only 500 levels instead of 2000 ;)