Jack is a wild card. They'll give him a shot and see how it goes.
If it goes well they can pull an Apple and say, "The founder is finally coming back!" If not they can quietly find a replacement and say, "Now Jack is going back to Square, just like they said he always would."
It's PR insurance, basically, which Twitter desperately needs considering they go through CEOs faster than I go through freelancers.
Spend any real significant time with split duties. Most of his investors would say "We didn't put $x in this with the understanding that you were going to be a 50% CEO/leader"
