Smaller units will require a smaller backup genset to run one at a time if you want to do that. (Inverter units are already very good with gensets, btw.) As others have pointed out, efficiency may be higher with the 1:1 systems. Some mini splits can cool and heat simultaneously transferring heat from one room to another, but these are rarely seen in residential situations because they are more expensive. It may only be available in systems with more indoor units. There is one downside to heat/cool units if you only need cooling. Complexity is increased meaning more things to break. I don't know if that is significant. I had a reversal valve fail under warranty which cost the installer, but not me. Note that if you have oil heat, the heat pumps may be a less expensive way to heat. Two 1:1 systems means another compressor unit sitting outside. If I had all 1:1 systems, I'd have seven compressors sitting on pads :-( You should know that the better mini splits have inverter-controlled compressors and variable speed blowers. This combination increases efficiency and comfort by stabilizing temperature and improving humidity control. Now consider this. A single unit that can throttle back to 30% of 6000 BTU can run at a 2000 BTU minimum. If you have an outdoor unit that can run 12,000 BTU your minimum becomes 4000 BTU. I believe that the smallest multi Mitsubishi compressor unit is 20,000 BTU so in this example, its minimum might be 6,000 BTU so you gain little from variable speed unless you are running both units at the same time. This would be enough to make me go with 1:1 in your situation despite "slight" increased cost and another unit to maintain. In addition to those arguments, if the price is similar, but you are running one compressor at a time most of the time, you get more longevity for the compressors.

Thanks, all. I found the detailed schematics of the indoor units and branch boxes (branch boxes drive the indoor units). Looks like terminals 1 and 2 provide the single phase 220V power supply. Terminal 3 is a serial communication line driven by an IC named HU2001R3. The receiver on indoor unit side is HI2003R3. HU2001R3 is driven by the NEC microcontroller uPD78F1164. I could not find the datasheets for HU2001R3, but it looks like it is a kind of parallel to serial converter and it uses a reference voltage (220V input voltage in this case) to generate the serial output. This means that terminal 3 is also operating at 220V. So, it looks like 3-pole disconnect is needed.

I'm not sure I understand how advanced you are in your planning, but some input: Ontario Hydro rates are here (may not be exactly right for where you are): http://www.ontario-hydro.com/current-rates Note the peak times change between winter and summer, and nighttime (from seven) to morning (to seven am) and weekends are all off-peak and lowest rate. Peak rate in summer is peak a/c time. So depending what your loads are, 'timing' your usage can make a big difference. Investing in some more sophisticated controls to favour using electricity at off-peak might be worthwhile, and possibly some 'heat storage' (e.g. heat your water at night, and even more complex arrangements). Since you won't have access to natural gas (the cheapest), what really makes sense is to figure out how well insulated you can make the house. The better insulated, the less need you should have for both heating and air conditioning. For southwestern ontario, depending where you are, the a/c season can be pretty short (although this year was exceptionally long), and with judicious use of ceiling fans, shade (in summer) in rooms where you spend most of your time, you may be able to get by with minimal a/c. Hydro's rates may seem pretty high but it's possible that some solar could make sense, if you can 'self-consume' all of your solar (which is always going to be at peak or mid-peak rates). (I don't know anything about feed-in tariff situation in Ontario). One comment, be cautious about putting a fireplace in the basement, draught/draw can be poor unless carefully designed. And if you go for a 'tight' house, they may recommend against wood-burning fireplaces.

Drats I can't be much help,I've never been able to fix anything without tools. Maybe someone can help him MacGyver his way through it despite not having tools.