Bhutto played the main role.GHQ was at fault too but Bhutto refused to talk to Mujeeb.
It is not right and contrary to the political conflict at that time, that Bhutto didn't want to talk with Mujeeb
Bhutto had forced Yehya he would talk to Mujeeb prior to the assembly session and cut a deal for the future constitution. Mujeeb wanted a session first and then negotiations on the floor. Bhutto's statement that "tangein tor dein ge" was when parliament was called before he had reached an agreement with Mujeeb. Bhutto's claim was that he is the sole representative of W. Pak and Mujeeb must negotiate only with him.
By the way, I fully agree with Bhutto's stance on this issue.
When Yehya, after meeting with Mujeeb, made a statement that Mujeeb will be the future PM, Bhutto had forced him to take it back threating with agitation in W. Pakistan and argued that It was a political and constitutional matter and yehya should stay out of it . (other w. pak politicians except NAP were with Bhutto on this matter). At that time the issue of two prime minister for each wing was discussed as well.
It is a now a matter of speculation that Yehya was sincere or not, or he was to create a wedge within AL to cut a deal on 6-points with Mujeeb ( and between politicians of two wings by this offer prior to a parliament\s session.
It was the claim of Yehya that his offer of appointment of Mujeeb was in lieu of some concessions on constitution issues -- It was a constitutional assembly that was to form a constitution . Selecting a PM was not its job before implementing new constitution)