As the title hints, I am using Hero Gandalf in order to get some extra info on the upcoming card and potentially play it as his ability allows.

In order to augment my card draw, I opted to run Expert Treasure Hunter, which allows to do so by gambling.

Combined with Gandalf however, there is no gambling in the process.

My question is, how does it work when there are two copies of Expert Treasure Hunter in play. According to my understanding it goes something like this: (1) given that both attached heroes have quested successfully, (2) you "guess" the first card, (3) pick it up, (4) next card is revealed due to Gandalf's ability, (5) guess the newly revealed card and (6) pick it up.

Is this valid?

Yes. You decide in what order Responses resolve, and since Gandalf's ability is a passive the top card will always be faceup. So you'll have the opportunity to look at the second one, no guessing required.

Combine it with Wizard Pipe and Hidden Cache for a cash engine. (Props to whoever thought of it first.)

Isn't this card errata'do to be one per hero? Either way with Gandalf you get a free card each phase, but you reference multiple copies so that wouldn't matter

They only changed that to keep you from throwing all your copies on a hero like Eowyn or something, who is going to do nothing but quest and frees up your other two heroes while still benefiting from multiple copies. If you attach one copy to say Gandalf and another to Elrond, as long as they're both committed to the quest and you're successful, both copies will resolve.

Thanks everyone for your help.

My question arose due to my belief that the "successful quest" condition ends once you use Expert Treasure Hunter's response, and thus a second copy does not benefit from that occurrence.

Combine it with Wizard Pipe and Hidden Cache for a cash engine. (Props to whoever thought of it first.)

This sounds really good! I will definitely try it once I get my hands on Hidden Cache.