We have to consider that there's Micky as well. If Micky stays, I can see both Gündogan and Hummels leaving. I mean, Gündogan was basically already gone last year and we even have his replacement already (Weigl/Castro and Merino coming). If Hummels wants to go to Barca/Real/anyone except Bayern, then he will be allowed to go considering what he's done for the club. Micky might be unlucky in this case and "forced" to stay, considering we don't really expect ridiculous sums if the Juve rumors are true.
However, if all three want to go and Hummels wants to go to Bayern, I don't see Watzke agreeing to this for the Hummels transfer. It would just be a disaster to sell 3 first team players, with the current captain going to Bayern. Even if we sign Götze, the mood will be that we might as well stop playing football at that point :lol