DUAL mode is like having two receivers, each independently tunable and capable of any mode. Each will tune over the whole 1kHz to 2GHz range.
The Diversity mode uses two receivers in a combined mode, each RX being fed from a different antenna. This can be used for diversity reception or to act as a ‘noise canceller’. I’ve found this mode very useful, if a bit fiddly. With care, it can get local noise down from S9 to S1 and the signals previously hidden by the noise come through.
One has two independent receivers in the SINGLE mode toggling between TUNER 1 and TUNER 2. It seems that the DUAL mode offers something else. But what? I will be glad to get an explanation from a SDRplay expert.