Turn both rugs over and align the edges that you want to join together so they are side-by-side and not overlapping. The row of squares on the edge of both canvases should meet evenly.
Secure the alignment by using a few strips of heavy masking tape or large, evenly spaced safety pins to join the backing squares of one piece to the adjacent squares on the other. Set the rugs aside and thread a large needle with rug yarn in a blended shade.
Use a whip stitch, starting at one end of the canvas, to join the rugs. Insert the needle and yarn through the first square on the edge of the rug canvas and bring it up, through the bottom, of the adjoining square on the other rug. Knot the yarn to secure it in place. Work your second stitch, and each stitch thereafter, through the next square in the canvas, inserting it in the square of one rug, and bringing it back up through the adjoining square on the other. Continue working the whip stitch until both rugs are joined and knot the yarn as needed on the back side of the rug. Cut off any excess yarn.
To secure your work for heavy use, tear off a length of heavy masking tape or carpet tape and apply over the length of your seam.