Both Slide and Hunter mountains can be done in a long day if you use the shortest trails. The trailheads are about 15-20 driving miles apart.
Slide from County Route 47 via Phoenica-East Branch and Wittenberg-Slide trails: Round trip of 5.5 miles, 1800' total elevation gain.
Hunter via Becker Hollow trail and a bit of the Spruceton trail: Round trip of 4.5 miles, 2200' total elevation gain.
There are couple of slightly longer options for Hunter but with less elevation gain:
via the Spruceton Trail: Round trip of 6.7 miles, 2000' total elevation gain.
Loop using Spruceton, Hunter, Devil's Path, and Diamond Notch trails: Round trip of 7.85 miles, 2000' total elevation gain.