I am wondering if you mean a cord that has a 30 amp plug on one end and a 50 amp MARINCO outlet on the other.. Many people get the outlet end of an RV power cord confused with a plug because it PLUGS ONTO (not into) an OUTLET..
But nobody will make a cord with two male plugs for reasons of safety. They call 'em suicide cords, and for good reason.
Slight retraction.> They do make a "Y" adapter with two 30 amp plugs and a 50 amp outlet,,, but there is no connection between the two plugs HOT pins, thus power can not flow from one of the 30 amp plugs to the other.. ALso the adapter is next to useless Id not buy one.