PDF Bookshelf lets you work with two open bookshelves at the same time.

When you open or create a second bookshelf, the titlebar in each window will be modified to show the name of the corresponding bookshelf package.

You can work with more than two bookshelves, but only two can be open at any one time. If you already have two bookshelves open, you will have to close one of them before opening another one.