If you are simul-climbing part of a route because it is technically easy (e.g., 5.4 or 5.5), you still might come across an isolated crux section that is two or three body-lengths and more difficult (e.g., 5.8 or 5.9). That portion might warrant a belay for the leader and the follower. Communication between the two climbers is key, and the leader should alert the follower when there’s a tricky section.
Setup The leader is tied in normally. The follower should be tied into the end of the rope with a figure eight, with slack coiled around the shoulder; then clove-hitch that slack to a biner on the belay loop. The rope coming off that biner should run through a Grigri clipped to the follower’s belay loop and up to the leader. The amount of coils on the follower’s shoulder is determined by how long the rope is and what distance separates the leader and the follower. The Grigri allows the follower to adjust the amount of rope out between the climbers, and it can be used to self-belay through short, tricky sections (called a "mini-pitch").
One When the follower comes to the crux, the leader should find a good stance and place a piece or two—this is the “mini-anchor”—clipped above his waist. Then he simply clips the rope through those pieces. If the leader is not at a great stance, he can clip in directly to the anchor with a sling.
Two The follower begins climbing the short crux section, pulling in slack through the Grigri as he goes. If he were to fall, the rope would come tight on the leader, clipped through his mini-anchor, and on the follower's Grigri, arresting the fall.
Three Once the follower is through the crux and back on easier terrain, the leader continues climbing above his mini-anchor. The follower will have a loop of slack (the slack that he pulled through the Grigri while climbing the crux), and he can either remain stationary and feed that slack back to the leader (as the leader climbs), or add that slack to the coils around his shoulder.