Stock control. Combined variations

I have a sailing club membership site. We sell tickets on our site and there are two different prices. One for members and one for non-members. The product is a single ferry trip, and the variations are members or non-members. Obviously the number of seats is at the product level not the membership variation level. We don’t care if seats are filled by members or non-members.

My problem is how to set the quantity available at the product level, then reduce that quantity regardless of whether the sale is to a member or non-member. I don’t want to set a quantity for each variation.