I was listening to the LCS Hockey Radio Show with guest Greg Wyshynski of Yahoo's Puck Daddy fame. The concept of using two goalies at the same time came up. There is a rule preventing such a ploy, which of course means someone tried taking advantage of this idea somewhere along the line, therefor forcing the rule.
According to Brian McFarlane's book "It Happened In Hockey" Rat Portage (now known as Kenora) Thistles tried this "at the turn of the century." No details about who the coach was or goalies were are mentioned, just that the coach "benched one of his forwards and inserted a second goalie in his team's net. There was no rule against it at the time, and the coach thought two goalies would make scoring almost impossible. He was wrong. The goalies stumbled into each other and left enough openings for the Ottawa boys to score. The strategy was quickly abandoned. A rule was soon adopted preventing a repetition of the ploy."