After taking a look, SpaceCadet is actually a superset of DualUse now, as far as I can tell. I made SpaceCadet work so you can apply per-key and global timeouts to the functions, and you can actually map any key to any other key (you aren’t limited to the certain modifiers). Of course, it doesn’t have the layer support, at least from an initial glance, but it may still support that. Personally, I’d rather have the main keymap separate, and then define my overrides like you can do in SpaceCadet, but I know others would have the opposite thought.
The two plugins definitely have a lot of overlapping functionality, but I am not sure if they have the same use case. DualUse looks to be a more powerful plugin in concept, with the keymap integration, though SpaceCadet is currently more extensible, but just initialized differently (you can map any defined key to any other defined key, and have separate per-key timeouts). Effectively, they are the same, but you have more control over end behavior with SpaceCadet. If you want to define your mappings and overrides in the keymap, use the DualUse style, and if you want to have an explicit list of overrides, you can use SpaceCadet. As I said, I prefer the explicit nature of mapping one key to another, so I am going to use SpaceCadet. You can definitely implement the SpaceCadet default Shift/Parens with DualUse if you are willing to change the keymap (and assuming you are using DualUse, you are likely changing the keymap), but you can also likely implement anything you wanted to do in DualUse with SpaceCadet (I am unsure about the layer switching, as I don’t use that, but I assume it’d be possible).
Even though they implement similar functionality, I don’t think it hurts to have both options, and I don’t see a reason that they’d need to be combined. Of course, if it is desired that they be combined, I wouldn’t mind implementing the DualUse keymap logic into SpaceCadet (though I sort of like the DualUse name more, so I’d combine and rename). 