Two combis win on:

1. Low capital cost of installation

********. One boiler with no pipework changes will be less than half the
capital cost of two boilers with substantial changes to pipework.

2. Running cost, upstairs heating can be off most of the day.

Subzoning is easily achieved using a single boiler.

3. No waiting for showers

Have you ever used a storage system? There is no wait for showers.

4. Showers don't run out of hot water

A shower will not run out of hot water, provided the usage is below the
capacity of the coil. An old cylinder may have a poor coil, but would still
hold sufficient water for the longest of showers.

5. Simple zoned heating system with independent time zones.

Subzoning is easily achieved using a single boiler.

6. Different rad temperatures on different floors.

Which serves what purpose when you have TRVs and subzoned thermostatic
control?

7. Less controls to go wrong, with accompanying down time and expense.
(no
zone valves and the likes)

But multiple pumps and boilers, all of which can go wrong.

8. Always heat and hot water. Two combi's means built-in redundancy.

About the only advantage.

9. Release valuable space for storage.

Where are you putting the new boiler? The cylinder and tanks are often in
otherwise unused locations. You'll need to run a new gas line as well. The
existing 22mm won't be enough.

Only disadvantage is slightly more expensive on regular servicing, which
is
overwhelmingly offset by the above points

Which is why every house in the country has 2 combis. The case is so
compelling.

Christian.