RRS 21.3 states that a boat moving sideways through the water shall keep clear of one that is not. This maneuver, often called crabbing, is very common at high levels of ILCA racing, and sailors are coached on how to do it. (See this video: https://www.youtube.com/watch?v=avH7S9ayFrI)

But what if two boats are crabbing, i.e., moving sideways through the water. This is very common on the starting lines of major ILCA regattas.

In those situations, my guess is that Rules 10-13 come into play; specifically, Rules 10-12 if the boats don't cross head to wind, and Rule 13 if they do.

But here's here's the rub: is "head to wind" based on true wind direction (the wind a boat sees when luffing, net of current) or the apparent wind direction? This matters because crabbing is most often done in light air, and moving sideways adds an apparent wind vector that pulls a boat's wind direction 10-20 degrees to the right (assuming they're starting their crab on a starboard close-hauled course and push their boom to the port side). 

Thanks in advance for your clarification.