There is a scenario where Frost coaches the bowl game for UCF, while also being the Head Coach for Nebraska in December, and prepping for next year. I agree that it would most likely be that Frost leaves UCF altogether and goes 100% NU in December, but he's not going to stay at UCF for the bowl game and not do any work for NU in December.
If UCF and NU agree to let Frost coach in the UCF bowl game, the timeline would most likely be like this.
Dec. 4th - Frost is hired by NU, arrives in Lincoln for press conference and starts getting his staff together for NU. He leaves a small staff at UCF to carry out bowl prep. The NCAA allows "split coaching staffs" in times of transition. UCF can also bring in interim assistant coaches & GA's to help out in bowl prep.
Dec. 5 - 22 - Frost and his new staff in Lincoln do recruiting for JUCO's and high school guys that want to sign early (12/20-22).
Dec. 23 - Frost hands off dead period duties to his new NU assistant coaches, and goes back to UCF to oversee the bowl prep.
Dec. 26-1 - Frost takes over the UCF football team and bowl prep for one final game with UCF.
Again, this is all dependent on whether UCF wants Frost to coach in the bowl game, Frost wants to do it, and NU is willing to allow Frost to coach one more game. I am sure NU would rather have Frost be 100% NU from day 1, but it might be a consideration they give to Frost during contract negotiations.