The best case scenario will be a rich man's Portland backcourt. Similar to Lillard/McCollum, IT/Fultz will be able to break down defenses with their advanced ball handling, above average passing, and reliable shooting range. Both players want the ball but that's fine because both players will be able to find each other and neither are afraid to shoot when their number is called.
The worst case scenario is something like Bradley Beal and John Wall. While those two are truly talented and have games that should complement each other's, it seems they often take turns rather than playing together.
Some nights John Wall is the orchestrator scoring and carving teams up while Beal sits on the 3-point line and ball watches his way to a 12 point night. Other nights play out like game 7 where Beal is engaged, on fire, running through screens and torching his matchup, all the while John Wall takes a back seat.
IT/Fultz will need to play together as 1 backcourt unit and not two individuals.