I have also seen cordless drills with two chucks that you can swap between by swivelling them round. This is very handy if you want to drill and then drive screws in quick succession because it saves having to swap back and forth between a drill and a screwdriver bit
I am not sure if you are referring to sets like this one or this . They often go together. In the former you can interchange drill bits, screwdriver bits or nut drivers by pulling on the collar of the knurled thing you see on the top right hand side, replacing the bit and ressetting the collar. It is great for quick interchanges even if you have a drill machine with a Jacobs chuck (which most do today).
In the latter this goes one step further bwhere you mount the drill bit on one side and the screwdriver on t'other which you see on the second device from the top. It mounts in the contraption above it and can be pulled out and re-inserted in the opposite direction allowing you to drill a few holes then insert the screws rather conveniently.
There is a third variation where this drill/driver combination is mounted on a pivot so the flipping action is even easier. However, the ones that I used (not the one here) was not quite as robust as I had hoped and I went through 3 of them building my deck. I don't know where the set is now, I thimnk my son has it or I could get a better picture.