Working both sides and joing second ball

I am completely stumped by something probably very easy.

I am working a pattern that requires joining a second ball of yard when shaping the neck. Center stitches are placed on a holder and pattern states to work both sides at same time after joining a second ball of yard.

How do I join that second ball of yarn?

thanks for your help!!

You just start knitting with the second ball of yarn on the side that doesn’t have the working yarn attached already. You can snug up the stitches and weave in the ends later. This gives you a strand of yarn for each of the shoulders.