Have one combi do the downstairs heating on its own programmer/timer and
one
do upstairs. Natural zoning, so you don't have to heat upstairs when you
are not up there. No external zone valves either, and simple wiring up
too.
Also if one goes down you will have another combi to give some heat in the
house and DHW too. Combine the outlets for DHW and all the baths you want
very quickly and no waiting. It will do two showers no problem at all. A
win, win, situation.

Two boilers?! Double service costs?! Double usage costs?! Naaah, think not.
That is the maddest idea I have ever heard.

When I refurbed my house I pondered for months what system to have.
Unvented... crap at filling baths unless you spend a small fortune.
Potterton HE... crap at filling baths (they run cold at half full!).
Conventional boiler and hot water cylinder... no good at filling baths
either.

In the end I installed a good combi and coupled it up to my existing DHW
cylinder. I run every tap off the combi but the HW cylinder feeds my showers
via a pump. That way I have just one boiler, endless HW at all the taps
(inc. the baths) and a shower that is more powerful than any combi can
possibly provide. A little more expensive to run but well worth while. I
also have the added benefit of storing some water in the loft tanks which is
damn handy if the water supply fails (which it has done twice this year
already!).