Why does it need to stay open? The Elanore Rosevelt Blanket has a Double knit center pannel, with a gartered Edge.
your blanket needs to stay open at the double knit section?
is it only a pocket then? or if you just want it to be a warmer section of blanket, you can make lice to tack it together.
To keep the sides seperate, you can use different colors, or just set markers at every so many stitches, and check each time you get to a marker.
I do mine just the same, to tack the sides together, and the changes make the pattern.