Two color split stitch / double split stitch is used to make two toned chains.
Thread the needle with two different colors.
Stitch a simple straight stitch. Bring needle up at 1 and go down at 2.
Now bring the needle up at 3. Make sure the needle splits the two colors like shown below.
First chain is done.
Repeat the steps. Go down a little further away.
Split the threads while coming back up.
Here is a trick to make this easier.
When the needle is going down, don’t pull it all the way through. Bring the needle up and then tighten. This will be easier to split the threads.