This sounds an awful lot like the former World Championship Wrestling's World War 3 setup. Their are three rings setup side by side and in each ring their is twenty superstars. The sixty superstars then wrestle for as long as is needed, until their is five superstars left in each ring where they move into the third ring to wrestle until their is one survivor.
This could be a possibility but would go against all former Royal Rumbles, excluding the 1992 Royal Rumble, where the champions would both be in the rings with the other superstars. I think you would have to have the two champions of their respective brands not be allowed to enter into the Royal Rumble. And, I think someone has brought up the scenario that it would not last the three or four hours you were planning on. If anyone remembers World Championship Wrestling's World War III pay-per-views they did the idea I proposed above, but also had anywhere from four to six matches along with it for the pay-per-view's entirety. The only reason they were able to do this is because at that time period, WCW had an incredibly large roster. WWE could only pull this off if they brought up some of their minor-leaguers in the Ohio Valley Wrestling promotion up to enter the Royal Rumble.