First question, where does 16kW come from.  It's a big loss even for a 220sq m house unless its poorly insulated or it gets very cold, which it may depending where you are.

Second question, what exactly is the buffer tank for, what do you mean by capacity in this case?

@jamespa The house is actually 280sqm but I wrote 220sqm as I'd just be conditioning that amount. It's not that cold here. Design temp is -3C. I've gotten several quotes and they almost all suggested 16kw though they use rule of thumb. Also my heat load calculation came in around there. The problem is that the ceilings are high (2.7m on the ground floor and cathedral ceilings on the first floor of about 3.5m).

I'd get the buffer tank to store water for space heating and DHW. Basically just an extra tank than what the heat pump can store.

OK.  Im not going to be able to answer the question, but can give you some pointers.  Others may chip in.

1. Forget the idea of a 100l buffer tank to help with capacity, it wont.  Even if you heat it to 70C it stores only about 6kWh which isnt going to make any difference if you are short of capacity in a prolonged cold spell.  You need a much bigger tank, eg 1000l, if its to be used for this purpose
2. You really need to get a better handle on heat loss if you can.  With such high ceilings ventilation loss is going to be a significant, possibly dominant factor and its a big unknown.  Here in the UK its frequently overestimated by a factor of 2 or more.  The standard assumptions for an 'older' house are 2-3 air changes per hour, but there is evidence to suggest that a more reasonable value in many cases is sub 1.  If your rooms are draughty then 2-3 may be right, but air changes happen at the surfaces (and particularly the edges) not through the volume so bigger rooms, and high ceilings in particular, will actually reduce the number of ACHs simply because there is less surface area for the amount of volume. 

   If you have some historic fuel usage do a sense check against that.    If you dont it may well be worthwhile getting a leak test done as well as a proper heat loss survey (which you can do yourself of course).  In the UK leak tests are about £300, dont know about Italy.  To give something of a feel, in my own case (200 sq m 1930s build with fabric improvements made over the years) I had 2 full 3 hour surveys (I paid for one of them) which came up with 16kW.  There were errors in the surveys, if I correct these but otherwise use standard MCS calculations I get to 10.5-11kW.  The loss determined from fuel consumption over 2 years is 7.5kW, which I can get to from fabric calculations by assuming 0.5-0.8ACH.  0.8-0.5ACH is actually very believable, the replacement double glazing has no vents, the chimneys are blocked, there is solid flooring over most of the downstairs, many of the walls have IWI and there are no detectable draughts.  Furthermore the house is essentially cubic, so the surface area in relation to the volume is about as small as it could be (barring a spherical house).

3. Sticker capacities are frequently not representative of actual capacities which generally vary with OAT and FT.  You need to check the actual capacity at your design outside and flow temperatures.  The variation can be either way.
4. Now to address 2 vs 1.  Obviously 2 gives a degree of redundancy but only against pump failure, not against power failure.  It also gives you better modulation ratio (ratio of min output to max output).  Most of the time you will likely be operating at about half the design load, so for this reason and to make redundancy work you need to plumb it so that either one can cover the whole house.  Several heat pump makes, including Daikin I think, support cascade working natively and I would check out the capabilities in case of failure.  Trying to find someone to rig up a bespoke control system will likely prove difficult, so unless you feel happy to do this yourself you want something that is designed to do what you want.
5. That said heat pumps are reputedly very reliable, and the cost of an extra unit is significant.  There may also be planning implications.  It would be unusual to bother with the heat pump redundancy and more usual to stick with a single pump, relying on the immersion heater for emergency DHW and any other heat source you may have (electric fan heaters, a log fire) for emergency space heating.  
6. Turning to the battery/PV system - the PV might not help much with your heating, most solar happens in the summer and most heating in the winter.  It should give you free cooling though if you design also to cool, as well as free DHW for 6 or most likely more months.  Im aware that the skies may be clearer in Italy winters than in the UK, so you may do somewhat better.  Check pvgis for predicted solar yield by month.  10kW (peak) is a good high number and with solar PV in general the more the merrier.  You need to check limitations on (and payment for) export if you plan to do this with your excess - you will have bags of excess in summer.  Your solar designer should know about the local rules.  If you don't get paid for export and you don't have a way to consume the excess, it may be worth dialling back the quantity.
7. I presume you mean 14kWh battery not 14kW.  If your heat load is in fact 16kW then thats probably about 3 hours when its running at full capacity, but then it rarely will be at this level so most of the time more like 6hrs.   14kWh is a reasonable number for load shifting to take advantage of ToU tarrifs and to help with self consumption of the solar if thats the objective (although its only 90mins of peak output, you are gonna need another sink or to sell back to the grid!).  I would personally recommend that you do some back of the envelope ROI calculations on the battery.  The environmental arguments for home battery storage are tenuous at best and the business case may be marginal, depending on what ToU import and export tariffs are available.  Personally I cant make a business case work for a battery, but I already have PV so I would be paying for a second inverter whereas you get it almost 'for free' as part of the PV installation (assuming you choose the right inverter)

Hope that helps, feel free to come back for clarification and others may well chip in.