You can't award a single bounty to two users, but you can always create a second bounty once you've awarded the first one, and then award that one to the second user. There is even a specific bounty reason for this:

![Reward existing answer: One or more of the answers is exemplary and worthy of an additional bounty.][1]

You will, of course, need a little extra reputation in order to do this.


  [1]: https://i.sstatic.net/rEgBE.png