Once the merger is finalized, they will be taking two entities that mirror each other organizationally in sales, marketing, product and back office functions and slimming them down to one. How they plan to execute this is anyone's guess, but I think it's reasonably to estimate that 40-50% of the headcount is going to be reduced. Given that you most likely have a territorial counterpart at McGraw Hill, I think only one of you will be sticking around in 2020. Have you asked about this at your interview? I definitely would if you are serious about the position. However, keep in mind that anything they tell you can and probably will change once the companies merge. The person you'll be reporting to may not be around in 2020. If you take the position, you should go into it with the realization that there is a decent chance you will be out of work in a year or less.
A very probable scenario that is predicted in this post by @Zcr0euj