Hello Krzysztof,
I've sailed many ocean miles with a 'boom' (which is often in fact just another batten) nearly 23ft long, including in F9 seas, and dipping it in the sea was never an issue. It's at an angle of ~10˚ to horizontal, which puts the clew higher than with bermudan rig. Actually, if the stability of the boat will stand it, a higher AR rig is more efficient, and the boom is then not appreciably longer than the boom of an equivalent bermudan rig.
The point at which two masts becomes a good idea is when a single sail would be too big to handle, at something over 500 sq ft and 30ft LOA. A Nor'sea 27 will be fine with a single sail.