Check out the Alfa Romeo TwinSpark concept. They reckon they get the same power from a 2 valve per cylinder engine with twin plugs as other manufacturers with 4 valves per cylinder and a single plug.

Since I did Chem Eng for my degree, if the twin plugs are correctly timed you would expect more of the fuel to combust in the mixture. If your valves are smaller, then you lose out on the volumetric flow of fluids into and out of the engine. It is the overall package characteristics which you need to maximise together.

The best combination will depend on many factors which are not easy to vary. The timing of the camshaft(s) and their lifting characteristics etc all play a critical part.

One thing is certain - the XF engine with twin plugs will burn the air/fuel mix better than an XF engine with a single plug providing the plugs spark together.