Oh, this is a modified Russian join!
1) find the end of both pieces. I’m using an off-white and a variegated green so you can easily see what I’m doing, but this can be used to join any two pieces of yarn that are the same weight as each other whether they’re the same color or not.
2) wrap the ends around each other like this. You don’t actually have to lay them down, I did it for clarity in the photos.
3) you’ll need a needle like this. They’re sold under the names “tapestry needle” or “yarn needle.” If you ever manage to find them made out of bamboo or wood, PLEASE TELL ME.
4) thread the yarn through itself as shown in the video above. HOWEVER, where that video shows strand #1 being threaded into strand #2, with the Russian join you’ll thread strand #1 back into itself, using the wrap around strand #2 as an anchor point. I use my toes as a “third hand” when doing stuff like this, but I’ve also seen people use the heel of their hand or a convenient paperweight or even nothing at all. Whatever will let you create a tight join is what you want to use.
Make sure you get this loop as small as you can.
5) repeat on strand #2. Trim the remaining ends.
6) you’re done! This is one of the strongest joins in crochet and knitting. It’s virtually undetectable in a finished project, and because of the wraparound shown in step #2 it can’t pull apart like a knot will.
7) don’t forget to put away your needle! “I’ll just stick it in the ball for later” is a comforting lie. The needle will disappear into the ball and you will never see it again, unless your foot finds it later. Don’t do it to yourself. PUT IT AWAY.