Hello Solo, thanks for the build update.
You can do exactly as you have said, plug them in to 2 separate channels, BUT you can purchase a mixing unit which basically plugs into the channel for the speedo and then the rudder channel and you plug the 2 spedos into the mixer, so you will mix both speedos with the rudder and you only use 1 channel on the receiver for the 2 motors ...see ?. when you apply rudder when moving one motor speeds up the other slows to assist in the turn. When stationary one speeds up the other goes into reverse to assist the turn, all very clever and they are not dear to buy mine cost £15 its a MTRONIKS W tail mixer, what speedos have yo got ? MTRONIKS speedos fit directly into this mixer etc.
Hope this assists
Sub