I can't be certain from your description, but it sounds to me like you have 1 BDR controlling the hot water valve to allow hot water into the hot water cylinder, and 1 BDR controlling the central heating valve to let hot water into the radiators. The valves control the boiler via the orange wires. Unless you have a BDR directly connected to your boiler (which is an option, but most people don't have), then don't tell Evohome to control your boiler - let the switched lives (orange wires) from the valves do that.