3 channels will suffice for any multimasted ship, 2-masted schooner to 4-masted barque, anyway: rudder, foremast, main+mizzen (or main+mizzen+jigger for a 4-masted barque). For sails, the rule is: 1 channel for masts forward of the CLR (center of lateral resistance), and 1 channel for the sails aft of the CLR. This allows you to use the sails to steer, a very necessary requirement if you want to both tack and wear, in my experience. Multimasted ships are not as rudder-dominated as sloops; you can't, under all wind conditions, simply jam the rudder over to head up or fall off. It's the misunderstanding of this point that has given the multimasted ships such a "hard-to-sail" reputation, I believe. They work just fine if you follow the rule I've given, and if you adjust sail area to match the wind conditions.
I've gotten away with 2 channels for a bottle boat 2-masted topsail schooner, but that boat requires more microseamanship (and luck with weather) than the 3 channel versions of sailboats. She sails, but her maneuevering is not as elegant as the 3 channel ships.
I don't think that 2 channels would suffice for a barquentine, Mr. Positive (love your miniatures). Fixing the main and mizzen sheets, and only rc-ing the foremast squares, will make it hard to wear ship, or make a run. Going to windward (with tacking) is all fun and good, but at some point in your cruise, you have to sail back downwind to your starting harbour
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My ships (gaff sloop, topsail schooners, brigantine, brig, 4-masted barque) can be seen here under my pre-mayhem-crash screen name of Brooks, or over on rcgroups.com (sailboats and scale sailboats forums) under Brooks.