A dual pot, also called stereo pot, is exactly a pile of two pots stacked together, that share the same shaft. such a pot has either a A (log), B (lin) or C (reverse log - very rare) curve, or taper :
A blend pot has a MN curve which means that when the shaft is centered, both pots are at 100% :
That’s how you can have both pickups together, fully open, with a blend pot, which is obviously impossible with a dual/stereo pot.
You should probably read this : Blend pot : how and why?