The rapier and dagger are Joseph Swetnam's favorite weapon combination; his next favorite weapon is the staff, which he promised to write about in a second book which was either never published or lost to history.
As with most fencing "masters" of the time, Swetnam regarded the dagger primarily as a defensive tool. It's lacks the reach to use for a serious offense -- particularly at the "true distance" that Swetnam recommends -- but it can hold a sword point at bay reasonably well while you attack with your own sword.
As with the single rapier, Swetnam suggests a pretty closed stance with most of your weight on your right (lead) foot. You then lean forward as far as you reasonably can without being unbalanced. Awkward at first, yes, but this puts your legs so far back that an attack directed at them is pretty futile.
Hold your sword low and slightly to your side with the point up in front of you. Hold your dagger straight out high and horizontal with the tip very close to your sword point. This position concentrates your defenses around your head -- a very sensible thing to do when you've stuck your head out as far as the basic stance puts it. You've made your head a very attractive target, but as we saw before, Swetnam likes to be able to predict where the enemy will strike.
The close points protect you from a "wrist blow", a quick downward cut to your face thrown from the wrist. Whether you have much to worry about from such attacks in the SCA depends on your local Kingdom rules and whether they allow tip cuts. You might see people trying to quickly drop the point and thrust to your face, too, so it's best to prepare for such an attack.
The basic defense against a thrust to your face is to slip your dagger under the attacking sword and lift it up. Naturally you should take this opportunity to simultaneously deliver a counter-thrust of your own.
I'll be discussing more of the basic sword and dagger defenses and ripostes as the week continues.