how much muscle have you actually gained since starting working out?...
i would say dont use dumbells (hold on now...) because using a bar will force your arms to stay in line with eachother...
pay very close attention to form and think through your reps, you need to feel the muscles work in the same areas on both sides... this will take some good mind-muscle connection, but also be certain that when you set up for excersises like bench, you are not just flopping down on the bench and whipping out reps (not saying you are, but i see this waaaay too often)... but make sure to tuck (or flare) both elbows, watch your feet are planted evenly etc etc...
i would say this would correct itself over time, given you havent already gained a ton of size...
i would hold off on the dumbells, as they will be more difficult to keep exactly on the same plane of motion... and if you really just have to isolate that inner chest, i suppose you could do db flies with one arm...