Mattress stitch

I watched Amy’s video and I understand how to do this when the v’s are side by side. I am trying to do a shoulder seam and I don’t understand how to do this when the v’s are on top of each other? Pretty soon I will be sewing the sleeves in, do you use the mattress stitch for this as well?

I use mattress–pick up a bar on one side and the two sides of the stitch on the sleeve.

Knitty has a good article on it here–scroll down for mattress stitching.