As seen in Simply Knitting
A fast and neat way to cast off for seams using three needles.
 Step 1Place knitted pieces together with the right or wrong sides facing, insert the third needle through the first stitch on both needles as shown. |
 Step 2Knit these 2 stitches together, pulling both loops off the needles in your left hand. You should have just one stitch on your right-hand needle. |
 Step 3Repeat steps 1 and 2 again on the next stitches to give you 2 stitches on the right-hand needle. |
 Step 4Take the first stitch over the second stitch on the right hand needle as you would in a normal cast off. Repeat step 2 and step 4 along the row and fasten off the last stitch as you would on a regular cast off row. |