So the bug was fixed some time ago, but it seems like it's back now.
You can get 2 rifts for only 1 rift's price.
1. You need to complete your first rift, and then close it when you are done.
2. After that you need to watch the timer, at some point around 0-1 sec left until portal ends, the rift will change from "the rift name" to "nephalem rift", and you click on it at that point.
3. You will be in a rift with an invisible kill meter, the boss will spawn after you have reached 100% (which you can't see).
When you complete this rift, start a new game.
Why would you want to do this?
Because if you are like me and low on rift key stones or simply hate farming them, then you can spend more time doing Rift's instead of bounties.