Is there any data to support or refute the hypothesis that sailing ships of the line were only complemented with enough gunnery crews to simultaneously fire 1 broadside but not 2?
If it matters for precision, let's assume British ships of 1700s.
The crew losses during broadside actions were so high that even if the ship of the line was able to man both sides at the beginning of the battle, its crew would probably be depleted an hour or two later.
Also, even if there were not battles, ships had crew attrition from just being at sea (scurvy, accidents). A ship of the line could hold max. one thousand man crew (often this number was closer to 500), with approx. 10 men / big gun you'd need 720-1000 men to man all guns (three deckers had at least 72 guns) -- who'd be left to handle the sailing duties? Also this number included the marine component, which was 20% of the crew for British and up to 30% for Spanish warships.