Their main positions are different, Hazard AML, Kagawa an AMC. and we are set to play a more outright 4-2-3-1, which could easily field both players, considering that Kagawa has met SAF personally, its more than a back up plan.
I know. Since we already have 3 good wingers though, I expect Sir Alex is only looking at them as attacking midfielders. Hazard has recently said he favours the #10 role, hence why I can't see him and Kagawa joining.