Your title says combi, so.. With combi's the most important figure is the flowrate. 11 litres/min is fine for showes and the odd slow filling bath. Here is a recent post of mine...
For an even better flow rate and cheap too for what you get, assess using two Worcester-Bosch Junior combi's.
For high flowrates it is cost effective to use two Juniors and combine the DHW outlets. Worcester-Bosch will supply a drawing on how to do it, or ask me here. Two Juniors are available for around £1000 to £1100 depending on what sized units you buy. They have 24 and 28 kW models, you could use two 24kW or two 28 kW combi's or one of each. That is cheaper than the Worcester HighFlow 18 litres/min floor mounted combi and can deliver about 21.5 litres/min and never run out of hot water. The highest flowrates of any infinitely continuous combi is 22 litres/min, which is the ECO-Hometec which costs near £2K.
Have one combi do the downstairs heating on its own programmer/timer (Honeywell CM67 or equiv) and one do upstairs. Natural zoning, so you don't have to heat upstairs when you are not up there saving fuel. The running cost will be approx the same as a condensing boiler heating the whole house. No external zone valves either, and simple wiring up too. The Juniors are simple and don't even have internal 3-way valves.
Also if one goes down you will have another combi to give some heat in the house and DHW too. Combine the outlets for the DHW bath pipes and all the baths you want very quickly and no waiting. Best have the showers on separate combi's. It will do two showers no problem at all.
The Juniors are not condensing combi's, yet overall heating costs will be equivalent to a one condensing boiler as the upstairs will not be heated most of the time.
A win, win, situation.
Its advantages are:
- space saving (releases an airing cupboard). Both can go in the loft, or at the back of the existing airing cupboard. - never without heat in the house, - high flowrates (will do two showers and fill a bath in few minutes, - No waiting for a cylinder to re-heat - Natural zoning, one does upstairs and one does down - hardly any electrical control work (running a wire to a programmers/stat and power to each, - simple no brainer installation, - minimal components used. - less piping used - cheap to run overall as upstairs would be off most of the time - etc.