25Avalon
Mine is Vaillant. I have 2 actually for each end of the house. (Don’t ask. The system was here when we got here) one is hot water on demand and one heats a tank of water. The only problem I have is having to top up the water every so often on the one that heats the water tank (which means climbing into the loft) which I think happens with most combis.. I do have them serviced yearly.
Are you sure they are both combi boilers and not condensing boilers? Combi boilers only supply hot water on demand they do not heat tanks of water and all the heat and expansion system is contained within the boiler casing.
Over the last 30 years we have had three waves of boilers. Our previous house was long and thin horizontally, one room wide and essentially a chain of 5 rooms. We had a boiler each end of the house, both identical boilers and both WB. The hotwater system one one was never reliable.
After about 15 years we replaced it with two combi boilers with condensing flues. Also WB and both completely reliable for the next 15 years we lived there. When we moved into our current house and had the heating renewed the heating installer recommended Vaillant, and we accepted on the basis that we would rather he was installing a boiler he was familiar with and liked, than one he might not be familiar with or not like.
Again, no tanks as everything is contained within the boiler casing