So I’m creating an helicopter (s-97 raider) and I was wonder is there a way too not lose RPM when you invert a motor.
You can try putting the two motors on the same vertical axle, and not the inverted motor on top of the first motor.
At least I did this a few times, I'm not sure it will help for your design.
In the following image :
Red is the vertical axis, static.
Blue is the motor 1. Attached to red.
Green is the inverted motor, attached to red.