Is SSP worth applying separately when I already use DAP?

DAP and SSP both supply phosphorus, so you would not normally double up on phosphorus for no reason. The difference is that single super phosphate also supplies sulphur and calcium, which DAP does not. So SSP makes sense when your soil or crop also needs sulphur, for example for oilseeds and pulses which respond well to sulphur. In that case some farmers meet part of the phosphorus need through SSP precisely to get the sulphur along with it, rather than adding a separate sulphur source. If your soil already has enough sulphur and you are meeting phosphorus through DAP, adding SSP on top just for more phosphorus is usually unnecessary. The right call depends on your soil test: check whether sulphur is low, and whether your phosphorus is already adequate. Decide based on your Soil Health Card or soil test and your KVK recommendation for the crop, balance phosphorus from all sources together so you do not overshoot, and do not over-apply.