Running two effects of the same kind concurrently can yield amazing results. Stacked fuzzes or RATs? I’m in heaven. Other effects work less reliably well in pairs. Two reverbs, for instance, can sound killer but can turn an otherwise carefully crafted signal to smog. Twin phasers, in my experience, can be counted among the effects that are delicious together. It takes just two simple one-knob phasers to get very weird. Build two phasers into one, though, and add a few extra tone shaping controls, and the weird gets weirder fast.