I was able to repeat this step twice in row to make my templar stay in-game for as long as the rift/grift is open.
Simply enter GR/Rift with templar (equiped with oculus ring for that extra dmg) then invite the second player into your lobby with also another Oculus ring equiped.
In Rift there will be 2 oculus ring on the ground while in GR it will only be 1 ( or my eyes aren't that sharp to see double ring stacked on each other). And in GR second player cannot enter because you started it before the second player entered.
The moment you return to town, the follower will leave. So stay out of town and you should be alright.
But remember, if you exploit, chances are high that you will get banned like back in the day they did with bloodshard exploit etc...