BB, can you say how you combine DHW from two combis to run a bath?
JohnH74, has both connected for all the house and finds no problems.
The best way is to divide and rule. One combis doing upstairs one down stairs CH. Total independent zoning. One doing one bathroom one doing the other. One doing the kitchen tap, one doing the utility. If one is down you have backup. And one does not influence the other when taking showers.
You combine the two for the baths only. Use two check valves and a shock arrestor between the valves and the bath taps. Outlet from one combi with check valve, otulet from the other with check valve. A tee between with a small shock arrestor on this line.
As JohnH74 says, it fills the baths pronto.
Price up two decent combis to do the above and then a system boiler and cylinder with the zone valves etc. The two combis come out on top in costs and installation time. Two of them could be in the loft or side by side or on top of each other in a cupboard.
CH zoning, instant DHW forever (great for body jets), and no spec taken up by large cylinders. ....and redundancy built in. Always heat and DHW in the house. Wiring is super simple using a programmer timer. Having adjacent sockets for each combi mean just plug them and then using wireless programmers timers no wiring is done at all. So, simple.
Why many installers do not do this in two bathroom houses is beyond me, as they are cheap and offer so much more.
JohnH74, take no notice of the fool from Norwich.