Generally, in a 2 zone (I have 2 of them), there is a master (at least as it was explained to me. I don't see the effect of a master, as if I have what I think is the master turned off, the other one still runs and cools fine).

What I do know is that one cannot be heating a space while the other is cooling a space. However, if you'll be only using them for cooling, that shouldn't be an issue. You can just put them in cooling mode and let them be.

Would both indoor units be located near each other, or one at one end of the house and one at the other? If you go with 2-1s then you'll need a location for 2 outdoor units. If they are far apart from each other, and you did the 1-2 unit, you'd be needing long lines run...

You will be better off with two individual units. SEER on my12kbtu unit is 25. HSPF is 12. On my dual unit the SEER is 16.5 and the HSPF is either 9.5 or 10. Substantial operating savings. The dual with a single compressor can do two, three or four Evaps because the compressor, condenser fan and evap fan are all inverter controlled, meaning true variable speed.

Get two single zone units. That way if one has a leak or a problem you will still be able to cool part of the house. We have one 3 zone and another 2 zone unit (2 outdoor units and five indoor units). Also make sure you get the heat function even though you already have a source of heat with your house. We were able to use it when we had a problem with our infloor radiant while we waited for the part to come in. There is a very small upcharge for the heating function and in the overall scheme it was very little money.

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 a lot folks, with all these information , I have some idea now. I'll make a decision Tomorrow, Thanks again. I'll report back Tomorrow.

Thank you again folks. I just scheduled 2 1-zone mini-splits to be installed early next week. with all the information I got here, I am sure I made the right choice.

Hello pfoley, may I ask how big your house is (in sqft), which configuration of mini-splits you got, and how much your operating savings have been? I am trying to convince a friend who lives in a large house. Thanks a ton!

Hi! Good discussion.

Mitsubishi has discounts right now. I don’t know if they really help the bottom line. I

am thinking of getting one outside unit, and getting an evaporator for the main area, and one for one bedroom. The house is a thousand foot small ranch. I do wonder if the outdoor compressor can be strong enough to add a third evaporator later on, when the basement gets a partially finished area? Hmmmn.