You might be able to enjoy the Gringotts ride at USO, then queue up to be the first to ride Hogwarts Express over to IOA. Depending on early admission crowds, this should get you to the Hogsmeade section at IOA fairly early. The train does not open until the other park opens, and sometimes they do not fill more than one or two of the three total cars (to build suspense? to control the flow?). This at least gives you a show while heading to IOA, although fleet-footed guests might beat you to Hogwarts Castle from the IOA turnstiles.
It also depends on the day of the week as to how many folk fill the parks. Weekdays seem less crowded.
Then, of course, you can ride the train back to London to enjoy the scenery and new scenes of the return.