not to mention the added mass will reduce the run time significantly that said you would need to maintain tension on the belts or bands making this blaster even heavier this would be a good thing for if we needed a high torque motor in this type of setup but we don't torque equates to how much mass can be tossed out rpm gives you how fast it will be going relative to how much kinetic energy can be transferred to the dart as in how much the dart slips on the flywheels as it moves through...RPM doesn't directly influence fps, per say. It's the speed of the edge of the flywheel. Since your flywheels are connected by a belt, the edge speed is the same on both. This has no advantage over simply feeding the dart in the larger wheels in the back, and has the added disadvantage of extra friction in the system.
The faster the flywheels are spinning the faster the dart will end up how much momentum the dart picks up is dependent on the grip and slippage. Torque is a matter of how much the wheels will slow down when they are hit with the load of the dart...