It may depend on type of work.
I think it was clarified bit more that in case of 2 good the same cards it still may be better.
If you for example use noise reduction than dual card setup should be faster. If you use simple timelines single card setup maybe more fluid. This is how I understand it, but I'm not 100% sure.
Resolve engine seems to be rather not clever one. If you send job to 2 cards it assumes they have the same capabilities. In case they are very different things go wrong as whole process always needs to wait for slower card.
With GUI card issue is that data has to come to it from processing one which is not that crazy fast, so you waste some time here.
It would be cool if Resolve tried first to use GUI+compute one and if needed then 2nd one. Then for rendering it should use different strategy trying to get max possible speed.