Depending on how one's "quilt" is made, most users can install blackened solid brass (non-rusting) snaps (backed-up with some THIN webbing material) in order to make a "reasonable" sleeping bag out of the quilt, or allow it to be snapped-into a poncho that has compatible snaps.
IIRC, the correct size of snap is the #2 size for interfacing with existing snaps on USGI ponchos; but might be wrong.
Snaps will never "seal" the bag/Woobie like a zipper will do, but most users can install snaps, with a little practice and the proper (and cheap) tools. Few users have a suitable sewing machine, and the skill/knowledge required to use it.
There's a million ways to modify/upgrade older ltems that have some life left in them. Go for it, if you wish; some trade-offs compared to modern items, and that's up to you, and your budget.
OTOH, if starting from scratch, and if budget allows, suggest skipping-over the cheap Milsurp items, and doing some research (and ultimate money-saving) in order to buy more modern items.
It's likely true that TWO GI Woobies are likely to be about as warm as ONE Wiggy's item (No Expert--Just a guess). OTOH, the TWO GI Woobies are likely to have far greater bulk and weight. Not to mention that the user will need to secure both woobies to each other AND the poncho, lest the unsecured woobies drift around and become useless as insulation. Most folks that have used a GI Woobie that was not secured to the wind-breaking poncho understand that the insulating Woobie mysteriously migrates, and becomes less insulating. I doubt most folks used the tie-down strings on the GI Woobie; they were there for a reason, but were a huge PITA to use, compared to (more expensive, HINT!) snaps.
More than one way to skin a cat, but suggest being aware of the differences between using re-purposed old Milsurp stuff, compared to saving-up and buying more modern stuff.
Room for both approaches, just be aware of the trade-offs.