I assume that a significant proportion of ID proponents come from a mono-theistic religous background, and they therefore assume their designer to be a single entity.
Those individuals when asked to consider multiple designers will probably reject the idea and come up with some reason for doing so (e.g. multiple designers are not necessary to explore the 'theory').
Since it is hard to point precisely to what it is about the Universe around us that screams 'I was designed', finding the conceptual differences one would expect from a multi-designer as compared to a single-designer hypothesis is ... well ... a bit tricky.
ID (for me) makes little sense in any case; after all if all complex entities must be designed, so was the designer ... then who designed it ... and who designed it's designer ... ad nauseum.
If we can get back far enough to find a complex, yet not designed entity then ID is, by definition, false.
And since that is the ONLY possible starting point ... ID is clearly false.