I have a great finish I use for this type of thing. Lay the panels out flat on a long surface and butt them together.
Take some long plastic pins, or use some scrap yarn in a contrasting color and baste across where the edges are pushed together. This will just hold them in place while you work and help you to get a smooth and even join.
Attach your yarn on the right hand end and slip stitch the two end stitches together to get the connection started. Then, working from right to left, loosely slip stitch the two panels together by picking up one stitch on each panel and pulling your working yarn through. Work loosely so that you don't get puckers along the seam.
This makes a great little decorative ridge along the join that I particularly like.
Hope this helps!