I've used the phrase "could be" in purely theoretical terms. The reality is that the world operates under any number of economic "imbalanced conditions" nearly 100% of the time. In fact, the day you think everything is "balanced" is a good time to be concerned -- because in economic terms, "balanced" and "stagnation" are pretty much synonymous.
As for 100% efficiency in growing lettuce, bullhockey.
I used the "100% efficiency" condition to help illustrate how the process would work if we were trading commodities or finished products that weren't as easy to produce (in terms of the labor requirements) as a head of lettuce.
If you were producing commercial bakery ovens and I were producing farm tractors, then doubling your output wouldn't be so easy.
Consider the question again, and for the sake of the argument assume that two heads of lettuce per week is your current capacity. How would you address the problem?
Better technology for all