Yep you can take your MEP rating before completing the ATPL groundschool. I know a couple of people that have done this.
But I believe that the reason most don't do this is because they tend to do their CPL first in a complex single at a cheaper rate. Then go on to do the MEP rating nearer the time of their ME IR skill test. Doing the MEP first might risk getting a bit rusty on it and having to repeat stuff later. I don't know though - haven't done it.
Personally I'm going to do a bit more of my time in twins and take a bit more of a financial hit.