Mike,
In case you didn't try it already I would suggest to re-bind both receivers when in dual configuration! The difficulty might be that there are now two receivers both believing they are "primary" ones. This causes telemetry conflict.
AND: make sure you really bind both separately: you must remove SRS cable from SRS Royal as otherwise you end up re-binding both receivers as "secondary" eventually. (as turning of receiver power from PowerBox will impact on both receivers
Regards, Ilkka