I'm doing this right now. I'm using a doubling stand because my yarn is on cones, therefore easy to do. If your yarn isn't on cones a double bobbin shuttle can be used, or the two strands can be spun on a spinning wheel.
Not sure why I'm being such a slug, but I've barely touched the looms for the past while. July is almost over...I really need to get myself in gear! Both looms have warps on them so the plan for tomorrow is to get cracking!
cotton doesn't 'full'. It will bloom during wet finishing.
To wet finish cotton I use the hottest temperature water, delicate wash/rinse cycles in the washing machine, tumble dry. For waffle I don't press.
Laundering instructions then advise to use warm water wash, tumble dry.
A plain weave hem on waffle weave will flare due to differential in take up so I don't personally recommend plain weave hems. Dimensional loss in waffle weave can be huge, up to 50% Depending on several factors, ie density, yarn size, etc.