I was in two minds about getting the Founder Pack, as I read in so many forums that you couldn't game share it. I only wanted to play PVE with my son, but would have to pay for two copies of a game that will be going free soon anyway.
Anyways, I bought one copy of Founders, to give it a try, and I can confirm that it game shares on Xbox perfectly. We both play PVE together on 2 xboxes, and we have earned over £50 worth of V bucks, in the past week.
I thought this might help someone, while there's still one day left on the Fortnite sale.