Hi Harry,
This is weird because I have had up to 4 Mpx digitals on 1 IPD rx output without difficulties. This also has been on 5 cell Nicads, and since the Rx servo power is supplied by common bus rails in the Rx, I would suspect the quality (resistance) of the Y lead. One point though, the plugs were the old Mpx style plugs.
Regards,

John.