Not for the purpose of discovery, yes, that's part of the issue, and the other part is not to attempt to trick the President into statements that do not match evidence that is already in hand but not disclosed to the court. No perjury traps allowed.
All this assumes that Mueller's word is good on both topics, and in fact it isn't good on any topic. He is a known liar who isn't at all above misrepresenting his intentions here. I wouldn't advise Trump to dignify this idiot charade with his presence.
Sept 1st he will end it. He has all the information now why wait on ending this farce? He wants to get Trump to set down and talk to him and get him on a perjury trap that is why he is why he is waiting. Remember Gen. Flynn the FBI agents said he did not lie yet Mueller still charged him with lying and threatened his son with prosecution. Trump would be a fool to set down with this crook that has no integrity. He will say one thing then do another just to get trump. Also just what crime is Mueller investigating or what crime is Trump being investigated for. Trumps lawyer should tell Mueller to stick it we will see you at the SCOTUS