Is a retreating blade stall a problem that can be overcome on dual-rotor helicopters?
Dual and intermeshing rotors also suffer from retreating blade stall at high forward velocity. But the effect of this is much less dramatic than with a single rotor.

Retreating blade stall causes a loss of lift on one side of the rotor: the single rotor helicopter will start to roll. A helicopter with two rotors that counter-rotate doesn’t experience this rolling moment, only an increase in drag.