I found this area not so long ago (just forgot to upload it, Fraps says it was... January-13 2015, 06:24 am? oops). It's my first time finding it, and it caught my attention that you need a party of at least 2 players (or a long-fuse bomb) to be able to open the gates with the treasure boxes.
Did I find something rare, or is it already known?
It's fairly rare, but I suspect that it's been in the game since launch.
That room in the image link is missing something - the rocket pups.
One of them was killed and dropped a poison vial for you.
"You use the rocket puppies' rockets (not there in the photo, they have been killed)"
Probably, I tend to panic and rush kill them at first sight :B
If I ever find any other interesting place, you'll be the first ones to know :)
It is relatively rare. We've known about it for a bit. The wiki is working on an extensive map project here. The project is not live yet and well, I can't work on the wiki much until next week. Feel free to add photos, maps, and guides to the list in a way that is consistent with other up-to-date entries.
As for the puzzle, you don't need two people to solve it, but it certainly makes it easier :P. You use the rocket puppies' rockets (not there in the photo, they have been killed) to bust the ghost blocks. Certain weapons (like a long fuse bomb as you say) will also solve it solo.