Synchronising two DC motors
Even with the same RPM, props are not identical so mechanically linking them or using putting encoders or tachs (which would require both a motor and a ESC that can accept them) won't work. I would use a yaw gyro or side slip sensor. Or trim carefully and rely on your horizontal stabilizer.

Note that brushless ESCs, especially helicopters ESCs have an internal speed governor that you could also use. Trimming still required though.