With the extra installation costs well outweighing any savings against a
larger boiler.

Also, having two boilers means that your minimum modulation gets worse. In a
marginal heating situation, bumping along the bottom at 4-8kW, you want the
entire load on one boiler so it doesn't have to cycle unnecessarily. With 2
combis, you could have 2 boilers cycling instead of one pootling along at
minimum, at much greater efficiency.

Christian.