I had to synchronize clicking with a friend and we had to try four time until we could reproduce … so I don’t have a screencast, just screenshot of the result:
Steps to reproduce:
- Have two people open a topic logged in as two different users.
- No solution has been accepted for this topic before.
- Synchronize (for example counting down) clicking the “Accept Solution” of two different posts.
- Reload the page to see the real result.
- With some “luck” two solutions will be accepted.
I don’t know if it’s possible with the same account. I don’t know if it’s possible if any solution has been accepted before.
Also I’m really sorry but we’re running v2.4.0.beta2 +123. 
Once I find the time to run the update I can try to reproduce again.