Have two down sleeping bags"
1. 3 SEASON-> 30 F. Western Mountaineering Megalite 850 fill (factory overstuffed to 20 F.)
2. WINTER-> -20 F. LL Bean 750 fill down bag, size Long
The 3 season bag has enough girth (hence "Mega" in the name) to accommodate my light down jacket and down pants. This has, in my experience, extended the range to 10 F. in my Moment DW solo tent. I'm sure I could easily go to zero with this combo and my light fleece balaclava.
*In cold weather I have always put my zipped up WPB parkas over the foot of my sleeping bag both for a bit of extra warmth and mainly to keep the foot of my sleeping bag protected from melting the frost on my tent's bathtub floor "walls".
My winter -20 F.Large down bag is also large enough inside to accommodate these down clothes. Hopefully I'll never need to resort to this but it's nice to know that it's available. Without the light down jacket I could place my -30 F. heavy down parka over my bag, tying the arms under my bag with some paracord to keep it over my torso. That would also likely be enough to make the bag at least another 10 F. warmer while also wearing the down pants.
Having the down garments do double duty is part of the way I lighten my load. For winter the down pants are mainly "camp pants" but also, on truly bitter sub-zero days, they can be worn inside my GTX hunting pants.