I did the twin flat sheet thing for a while, until I weighed it--way heavier than expected! Now I only use it for car camping.
I have a 0° Kelty Coromell (semi-rec) and an 18° Sea to Summit TK II (mummy). Both were clearance buys. Wish I would have bought a spare Coromell when they discontinued them. So comfortable, but too hot, even unzipped, above about 45° (and I'm a cold sleeper).
I use the TK II above 40°. It's quite a bit lighter and half the bulk, but if I had it to do over, I wouldn't buy the TK II again. The material is too clammy, and the temp rating is too close to what I already had.
Using both bags, the mummy TK II works great inside the semi-rec Coromell. Perfect for below 10°. It changed my mind about needing a dedicated below zero bag. I slept great down to -27° in a tent with no stove.
I have learned the hard way that when it's below freezing, what's under you is more important than what's on top. Frozen ground sucks heat way worse than cold air. An insulated inflatable pad on top of a Ridge Rest is my go-to for anything below freezing. 2 Ridge Rest pads below 15°. A thrift store wool throw blanket folded in half directly under my bag anytime I can justify carrying it.
I also use a silk liner. It works well to regulate temp (like a sheet) when it's warm enough to have the bag unzipped and flapped open. Still wish I had a bag with a higher temp rating for 50°+.