Don't worry about the border crossing. You drive across the bridge and stop for 3-4 minutes to get documents stamped.
There is much more to do and see on the Argentinian side of the falls. You've kind of booked your flights backwards for spending the most time on the AR side. I'd spend the first afternoon on the AR side, spend the night on the AR side, the next morning in the AR side of the falls, and the second afternoon on the BR side of the falls. Then you're in BR for your flight out.