Just MHO. The Boss' Boss knows the Boss is a POS and what to reassure you that you are going to be around. He/She probably wants to hear these things WHILE the Boss is out of town - this is no coincidence. So if I were you, I would "show my hand" to use a poker metaphor, only enough to let the Boss' Boss know that things are not right with the Boss. This will be a tightrope walk - you do not want to start spewing out months or years of resentments, which will help no one.
IMO the Boss' Boss is on a fact finding mission. Give him/her some facts that can be confirmed and acted upon, while at the same time remaining professional and not acting like you wouldn't **** on the Boss if he was on fire (even though that may be true).
Also please be sure and answer ALL parts of his/her questions - such as where do you really want to be in the future and where do you see your career goals, etc.
This is all good news, IMHO. This is opportunity knocking! Please treat it as such.