The problem I found with bags that attach to each other was they were floppy and sloppy.
I find it easier to have a larger bag I can put the other gear into if I must.
Most of my stuff for winter is bulk insulation. Stuff like extra clothes, parka, sleeping bag and so on.
Summer is not a huge problem here. The only extra stuff for summer might be a floppy hat, bug repellent and spare water.
_________________________
May set off to explore without any sense of direction or how to return.