Here's a demonstration of splicing yarn, one key to a quality end product. Short of unplied yarn, the fewer plies and more natural the fiber, the more invisible the splice. When you knit the spliced bits into your work, they will be almost unnoticeable, especially as compared to tying a knot or knitting both strands of yarn together which produces a lump in the knitting. Splicing is next to godliness.
First untwist the plies for 3-6 inches. Then take one ply from each strand and twist them around each other like plies of rope, leaving the remaining ply dangling. Use both hands. Tie the loose ends together loosely. Smooth the yarn before knitting.
Tie the short ends of each remaining ply together loosely. Now the next ball is ready to be knitted invisibly into your work. While you knit, weave in the remaining ply by passing it behind the knitting yarn as you knit each stitch.