No, you can't have two brush libraries loaded at the same time. There's something you can do, though, to make loading them easier and faster:
Drag any brush category icon onto the Painter screen. This automatically creates a Custom Palette named "Custom 1".
Load another brush library, and drag one of its brush category icons onto the "Custom 1" Custom Palette.
Now, when you click one of the icons, that brush library is automatically loaded.
To rename your "Custom 1" Custom Palette:
Use Window > Custom Palettes > Organizer. You'll see "Custom 1" listed. Highlight "Custom 1", click the Rename button and type a unique and descriptive name not already used by Painter.
To save your new Custom Palette permanently:
Use Window > Custom Palettes > Organizer. Click the Export button and save your Custom Palette with the name you gave it. Now, if you have to reinstall Painter 7, you'll still have your Custom Palette to use again.
NOTE: Be sure that if you have to reinstall Painter 7, you first move any files you created (for instance, your new Custom Palette) into a backup folder somewhere outside of the Painter 7 folders.
You can create and save multiple Custom Palettes to use whenever you need them.
To open a Custom Palette:
Use Window > Custom Palettes and check the name of your Custom Palette in the flyout menu.
Use Window > Custom Palettes > Add Command to add main menu and palette menu command buttons to your Custom Palettes.
If you upgrade to Painter 8, you'll need to install the Painter 8.1 patch to have Custom Palettes as they were not included in Painter 8 (amazingly enough!).
Please do not PM me with Painter questions. Instead, post them here where everyone can benefit from them. Thanks!