Having a manager and a Director of Football is generally a recipe for trouble for the reason you give. In the case of Mourinho, moving in Grant and expecting Mourinho to yield some decision-making was an absurd thing to do.
However, in this case there may be a good rationale. VB's weaknesses are probably a lack of top-flight experience and the difficulty of commanding the squad and standing up to Abramovich. I think Hiddink could help with both. If VB accepts him as part-time Chairman to his CEO then the two could make a great partnership. Hiddink seems not to want and seems unable to be a full-time club coach but who better as a guy to help when things get a bit rough. Also, Hiddink seems to carry as much weight with Abramovich as anyone. He could be an excellent mediator between manager and owner if and when things get a bit sticky.
Naturally I hope for and look for the best outcome. But I genuinely believe VB having Hiddink around looks like a big bonus, where a more accomplished manager having another guy hanging around would be a problem.