If the OP starts weighing the gear, he will find that the foil is light and the other layer is somewhat heavy. The combo will weigh near as much as a more expensive, light sleeping bag. The military offers a poncho and a thin, synthetic insulated blanket called a poncho liner. The users there tend to be quite fit and experienced.

I've spent a night out in clothing with a balaclava and a mylar emergency blanket, not those mylar bags that are now available. I did a solo kayak trip on New Year's Day then expected to hitchhike the shuttle. Hitchhiking after dark on an Apache Indian reservation didn't work. I walked until midnight, found some wood for a fire, then laid on the ground until daylight. I survived that lesson and do not recommend it to others. Wind blowing on plastic sheeting feels just like cold plastic against you.

I've tried the OP's question and found the gear lacking. We are now back on topic, if anyone else wants to comment on mylar and a sheet as bivy gear.