Congrats on learning to fly and wanting to advance. IMHO....For the best success with twins, stay away from glow, change out the RCGF engines, because they are poor running engines as likely to fail as a over heated glow, and a MUST have is TwinSync. It recognizes an engine loss from the rpm's and brings the remaining running engine to idle so that you have more of a dead stick for the whole plane. Once you figure out which engine isn't running, then you can apply some power along with opposite rudder to maybe get the plane back to the field. Most non-full scale twin rc pilots hit VMCse and drill the plane into the ground. Not sure if it is still around.
https://www.wikesrc.com/TWINSYNC-W502.htm
Getting ready to fly the B-25, unless it is an old H9 kit, is probably a kit built plane and heavy. I would recommend flying some other 50cc-G62cc war bird with a high wing loading, such as a kit built P51, not the ARF, due to it being too light. Learn to fly a high wing loaded plane. You can also make your current plane act like it is heavy by never going over half throttle for the whole flight, including take off. Then you will get an idea of a heavy wing loaded plane.
Most likely, you will have to build another twin, and maybe build another not so detailed and just a basic B25 and learn on it.
Again, this is just my humble opinion from being in this hobby for decades.
Here is a forum on the RCGF 15cc engine. I have not read it, but I am sure you will find all of the different trials and tribulations of the engine and it's issues.
http://www.rcuniverse.com/forum/gas-...on-thread.html