Hi, my name is Aaron, i am an aeronautical engineer from Mexico.
First of all, it is a great project. The propellers have an RPM speed where the efficiency is maximum, the engines usually doesn't go at the speed desired for this efficiency so you need a gearbox to make this happen.
because of the speed, the gears must be helical, which are a little bit harder to machine but offer great performance at top speeds due to the fact that it has more contact area than normal gears, distributing stresses. These are common in high speed applications.
There is also a change in the direction of the power, the axis for the engines are placed horizontally, and the propellers vertically, so what it is need is whats called an spiral bevel, which can be used to decrease the rpm while its helical shape gives it durability.
you need an appropriate use of bearings for every rotating part and the use of some analysis tool to design the frame and rotating components to hold everything together safely and prevent it from Natural frequencies, which is the frequency at which a system tends to oscillate due to aerodynamic and inertial forces.
At first sight these appear to be the primordial aspects of design for this project.
Feel free to contact me at anytime, i hope i can be of any service for this.
My best regards.