For asymmetrical -- yes. If it is symmetrical, then you can get away with one, if you design for it. When it is leeward it is being pressed...
I have no doubt that is true; in fact, I said as much in my original post. In practice, I will probably pick an appropriate symmetrical foil, like...
I am building an 8' dinghy with leeboards. (If you hate leeboards, don't worry, I am also building a different dinghy with a daggerboard). I have...