Double GRGs can even appear in a single player game, if you do it just right.
I was doing a level 30 Greater Rift. I failed to summon the boss before the time ran out, by about 2 seconds at most. (It was very close.) I was fighting an elite pack when The Binder spawned. While fighting the spider, I ran around to a better position. When I picked up the purple globes from the elite pack I’d been fighting, a second Guardian spawned, Saxtris.
Tsarnis: Thanks for the reports about this. We are indeed aware of the issue, and as other players have mention this is in our known issues list. I went ahead and edited the entry in known issues to indicate that this can also occur in single player games under certain circumstances.
It’s not all bad, since all of the GRGs drop just like normal, so in a four player game you could be looking at 6-8 legendary items when (if) you survive the battle. Some funny footage of the mad scramble created by four GRGs appearing at once can be seen courtesy of Twitch streamer and King Leonidas cosplayer MICHAEL13785.