Promiscuous Pairing

If your team already does some pair testing, take a look at promiscuous pairing.
"Promiscuity, it turns out, is a good way to spread a lot of information through a group quickly. Rapid partner swapping ensures that a good idea, once envisioned, is soon practiced by every pair. Replacing individual accountability with team accountability empowers each person to do those tasks at which he excels — and allow someone else to take over for his weaknesses."

Try 60 to 90 minute sessions where you alternate partners each session.