Dave shows how stacking two differently tuned slap delays creates a wider, thicker vocal sound than either delay can produce alone. He is not only using two different plugins, but also sets up one of them (Tel Ray Delay) into dual-mono. So you get two mono versions you can additionally play with. Dave decides to use one side more as a doubler, the other more as an echo. If you can’t set a plugin to dual-mono in your DAW just use it on two separate aux tracks.
When all slap delays run together, the interaction between the asymmetric signals produces a spread that none of them could generate on its own. Feel free to experiment with it. Put it on guitars, synths or anything you think a slap-delay would add something nice to it. When you put this in the mix, it just grows.