I live in wicked cold zone here in VT. Last week we had -27* in the area. I run two Fujitsu mini-splits that are rated to be able to produce heat to -15*. Our state's electric Co., GMP, was incentivizing these units for heat and AC, via a lease or own program subsidized by them. I had to get 2 outdoor wall bracket units, one for 1st floor bedroom and one for 2nd floor. They did not/ have not yet figured out how to combine 2 cold climate heat pumps into one outside unit, UNLESS one opts for a unit that is only capable to 0*, not -15*
These are all electric, run on 240V, have inside distribution ductless, remote control wall mount/ hand held thermostats. I use them to supplement an existing propane condensing boiler, a Buderus GB 142.
They are very quiet inside, and provide excellent AC output and steady/smooth supplemental heat when it is extremely cold outside.
Note, I have done as much tightening of the exterior/interior envelope as possible with rigid foam board, foil faced on both sides, blown in cellulose to R-50 in attic, etc. The blocking of air infiltration has made the most difference in comfort level in the entire house, especially at the sill plate/rim joist area where the heat loss is often the most evident. My propane boiler serves the entire first floor with radiant, and the pre-existing hydronic baseboard throughout the rest of the house, basement and 2 heated garage bays.
I mention all of the above because mini-splits for those running electric households may be a good way to heat and cool their homes going forward. This 'cold climate' mini-split tech is widely used in Europe as well as becoming the latest efficient technology for many parts of the US.