Lay the carpet from one room down, stretching the excess into the doorway. Do the same for the other room until the two separate pieces meet in the doorway.
Cut the excess carpet with a utility knife, leaving no space between the two rooms and aligning the two edges as seamlessly as possible.
Lift the carpet and lay a strip of seaming tape between the two pieces with the adhesive side facing up. Heat the strip with a seam iron.
Lay the carpet back down quickly, and ensure that the two pieces are tightly butted together on top of the seam tape.
Roll over the connecting edges with a rolling pin to ensure they are adequately pressed down.