What are the two airlines? Do they have an interline baggage agreement? Are you doing just carry-on or can you check your luggage all the way through? If not, you'll need to leave security, claim your bags, go check in again, and then re-enter security. I did this once, and only once, leaving two and a half hours between flights at SEA. The two airlines I used did not have an interline baggage agreement, and due to delays getting luggage off the plane and to an exceptionally long line to check in at my second airline, I almost missed my flight.