Yes, I wondered about the two holes. Even if two species lived in the walls, why would the second one to emerge do the work of boring a new exit? Thanks for the ID.

When there are two insects inside a monothalamous gall, they are not necessarily in the same chamber, especially if one is an inquiline -- just taking advantage of the gall housing but not necessarily killing the gall inducer.

If you cut some galls open you'll see there's generally an egg-like, well-defined chamber where the gall inducer develops, and other space inside the gall around the chamber where other insects can potentially develop.

Because the holes are two different sizes, I would guess that two different species of insect emerged from this gall, and they likely had little contact with each other while in the gall.

Here's an example of an H. pacificus gall that had its shape altered by inquilines:
http://joycegross.com/images.php?img=2222_2337
The larger hole towards the bottom of the gall is in the area where the gall inducing cynipid normally develops. However, the top of the gall where the smaller holes are is abnormally expanded, and there are exit holes where inquilines (also cynipids in this case, but ones that can't create their own galls) emerged. Each wasp had its own private chamber inside the gall. I believe a paper was published about H. pacificus and these particular gall-shape-altering inquilines, but I can't seem to find a reference to it right now.