Your 120 is A big plane and designed for stunts. With two elevator servos you can match the travel using your radio and A degree meter, I make my own meters. With two back there you can keep your control rods short and close to the controls, that makes them stronger and easy to adjust. When you trim your plane you can adjust the throws with your radio so your plane will loop and not roop. If you kill A servo you still have an elevator working. The reasons for them can go on and on. Another thing to think about is you are installing A nice heavy gas engine. Right up front you know your plane is going to need some tail weight.
The aircraft set up is the most important part of the building next to building straight with correct incidence. Anyone can glue wood and sand.
I always build my plane and while it is in the bones I assemble it. Mount the tank over the CG and only install A servo if it has to be in A certain spot. Then I move all my gear around and hold everything in place with tape until I find the correct CG then hard mount close to those locations.
If you mount your gear now you will be adding lead to get your CG. I know someone has posted A thread on aircraft set up in A build thread someplace on RCU so you may want to do A search. It's the part of building that takes the time to learn. Take A look at some IMAC and 3-D planes or just some good weekend stunt flyers at some local fields and see how it's done. I can't remember the last time I built one of my own planes with less then 6 servos. I'm just finishing up A little SIG Kobra for A friend and customer for racing, as small as this plane is it still has five servos.
Set up makes your plane fly as well as it ever will!!