what exactly is it. do both pickups work or do you choose one or the other. Whats the advantage.

Both work together. Put one on my last boat. Didn't see any difference from the side water pickup drive it replaced.

I agree with US1 Fountain as I did the same. As a matter of fact I have two for comparison. My port is a '95 side intake and my starboard is an '02 dual pickup. One disadvantage is that the dual pick up will suck up sand and mud easier and quicker that the single if you get into shallow water . Are they worth it...not to me

I'm assuming the low water pickup is if your X is set way high. Can you block off one pickup?

I've seen the side pickup holes plugged with NPT plugs before so they don't suck air.

I'm thinking more like plugging the lower pickup to keep from sucking mud.

welded it shut. Not sure if something like Marine tex will stay in. Thinking it needs to be a permanent fill.

How about if you had the thru hull pick-up and the nose cone pick-up hooked to a Tee connection in the engine compartment? Then you could switch it from deep water to shallow water. Obviously the valve would be open for the hull mounted pick-up in shallow water then switch it to the outdrive pick-up in deeper water.
Rush

No reason to run a T if you've got a hull pick up. If it works in shallow water it will work in deep water.

I have a hull pick up for the engine and a DWP on the drive. The water that comes in the drive dumps out the side of the boat. I had side water p/u's on the drive before. I get a lot more water out the side dump with DWP drive. The X is a little high, but nothing extreme.