As long as there is any reasonably discernible movement of air, a sailboat can sail. In light air, the boat might not be going very fast, but it can sail. The decision as to when you should continue sailing and when you should give it up and start the motor is primarily governed by your own time parameters, i.e., how much time are you willing to spend in travelling a short distance? Sailing in light air is a special skill that many sailors have not yet learned. but people who know how to sail in light air enjoy it for the challenge.
Generally, a sloop is designed to sail most quickly and efficiently with both sails. However, depending on the conditions, it might sail to your satisfaction with only one sail. In a strong wind, it might actually sail better with only one sail. In that case, using both sails might overpower the boat and make it difficult to control.