It is...

The facts vary in the details. Sometimes Jason says something that sets off Anne. Sometimes she changes him without thinking. Sometimes One or the other notices right away. Sometimes the first notice that Jason has become Sonja is when Anne tries to change Jason to Sonja and finds it already done. Sometimes the change is as soon as they meet. Sometimes it may be hours later. Sometimes...
But it keeps on happening.

"This is just not working!" Jason almost bellows. "I'm just not going to get the time I need, and it may not work anyway. Are you ready to split me yet?"

It is far from the first time they have had this conversation, but this time Anne can finally say "I guess so. I haven't found any more flaws, and it should work..."

Jason of course does not listen to all the "maybes" in that statement. "You can? Great! Do it!"

"Now it's not going to be quite that simple. I"ve got to be able to concentrate on this spell. It could be tricky if I am not careful."

Jason sighs "...All right. I can wait a little while. But let's rush to some quiet place."

Anne isn't that interested in rushing, but there is no reason not to head to her home, and her room. There she can warn her brother not to disturb her, and put some wards on her door and window. Finally to Jason's relief, she is ready to start.

The first part of the spell is easy. To one side goes Jason's male aspect. To the other goes Sonja's female aspect. But there are so many other aspects. For many she must decide if they should be Jason or Sonja. Often enough they must be both, and that causes extra complications. But finally, Jason and Sonja are different people.
So far, so good. Now, she needs to make sure they stay different people. Right now, they would merge if they touched, and all her work would be for nothing. Now to make sure that doesn't happen...

There! Finally done! Now she can step back and admire her work, and Jason and Sonja can say, almost in unison, "Wow, great job! Thanks loads!"