Two Optis tack at once

The situation

On a big shift, two boats near each other both tack onto the new tack at the same moment. As they come out of their tacks close together, who had to keep clear?

The question

When both boats are tacking at the same time, how do you decide right of way?

The ruling

Rule 13 has a tie-breaker: when two boats are tacking at the same time, the one on the other's port side keeps clear. So whichever boat has the other off her starboard side has right of way; the boat that finds the other to starboard must keep clear until both are close-hauled. Once they settle onto the same tack and one is overlapped to leeward, rule 11 takes over.