Ideally, every pair gets their own space to work -- I'm not a big fan of open-plan offices. But, if you have to go down that road, you need to put some strict policies in place to ensure that the small meeting rooms don't become offices:

1. Rooms are strictly first-come, first-serve.

2. At the end of the week the room is cleaned and everything in it other than the base furniture is thrown out. Period.

3. The focus rooms provide no storage (no drawers or shelves). Just a desk, a whiteboard, two chairs for pairing, and either a display or a workstation.