A lot of bodybuilders prefer dumbbell bench presses to the barbell version because of the extra range of motion, but with the barbell version you can isometrically push your hands inward for better pec innervation.
With dumbbells it’s more of a balancing act, and if you drive your hands together, they actually move together and the dumbbells meet over your chest. Many trainees do that to flex the pecs on every rep (or to clang the dumbbells so everyone looks at them), but I contend that all you’re achieving is activating more of the triceps. You actually lose tension on your chest through the top third of the stroke as your hands move inward—until the ’bells meet and you flex your pecs.
If you want to keep your pecs under stress on dumbbell bench presses, try driving the ’bells straight up, with enough space between them at the top so your hands are slightly outside shoulder width. Of course, if you’re using 100s, your hands will be outside your shoulders even when the ’bells touch, but if you’re using ’bells that allow your hands to move in, don’t do it. Maintain the stress to get pecs that impress.