The practice of butt forward really comes from the adaptation of military scabbards into the civilian world. There is a photo of Hickok in 1867 wearing what may be a flap style scabbard and it appears that the flap is folded back, tucked into the belt behind the scabbard (it is difficult to ascertain with full certainty due to the graininess of the photo). In that same photo, Bill is wearing only one Navy revolver worn on his right side. Even tho Bill was noted to employ the reverse plainsman's twist draw, in this particular photo it does appear the revolver is in a position for a cross draw. It would be rather odd to have a military holster made to be worn on the left side since issue side carry was on the right side. Perhaps a custom made rig of some sort.
A photo of Bill about the time of the Tutt shooting, shows another odd looking holster on his right side, almost looks like it has a straight top, again, hard to see exactly. The pistols are worn butts forward but so high on his waist that one would have to think that he would have to employ a cross draw to get them into battery. He was known to do that when sitting down.
Two pistoled shootists were not that common, even in the day of cap n ball with notable exceptions. Even less common with cartridge revolvers. Damn things are heavy after a time. IMHO, wearing revolvers in 2 right side holsters, while not PEC, was possibly done but apparently not photographed a great deal. Fellers probably used what they could get there hands on that fit their needs at the moment.