This I just found out through an accident in the game but it works! I hope it works for you guys.
I had testingCheatsEnabled on and I set their motives on to static which will drain their magic for 5 hours and it will never replenish but they can still cast spells.
This only works if you have two witches and they are both high enough level to spellcast duel - that's not too high a level though, I think it's 2 or 3. If you want them to level up really quickly with no effort try this:
1. Have Witch A challenge Witch B to a spellcast duel.
2. When you can see in Witch B's queue that Witch A is approaching her to propose a duel, put another action in her queue immediately after that one, ideally one that can be performed indefinitely.
3. What will happen is that Witch A and Witch B will agree to duel but Witch B will be forced to complete the next action in her queue before starting the spellcast duel. Which will leave Witch A waiting for her to finish her activity. While she is waiting the skill bar will appear above her head and continue to fill until the duel is over, but since the duel is being postponed indefinitely, it will just keep filling and filling.
I hope this works for you guys, it worked for me but that could have been a temporary bug or something. I'll test it a couple of times and see if that's the case.