A-sails, syms and tackers
All A-sails are more efficient when used WITH a pole once you are sailing deeper than a beam reach. Having said that, Cruising spinnakers (and most are of the "A" variety) are designed to function very well without a pole-to the point where it is assumed there will not be a pole when they are designed, but that does not mean they are not even better with a pole... but that is enough scoop on that topic.
The Tacker. IAs much as I like Etienne, I share the popular feeling that they offer very little help, and in some conditions or points of sail, are actually slower-this is all in the context of their designed use: with A-sails. They were not really designed for use with sym kites-
Of course I understand that if you don't have an A-sail, and don't have a pole, but do have a bag or 2 with sym kites down below, you would rather fly those than nothing, and they would be better than nothing as long as you are not too close to the wind, and mind the sheet angles so the leech is not choked off by having the lead too far forward. Being symetrical, they do not have a luff and leech-each shaped appropriately-as an A sail does. Without a pole, you really just have 2 leeches, so your leading edge is going to be pretty critical (like hoisting a genoa by putting the leech along the headstay and flying it backwards-not very efficient). The pole serves to "change" the shape of the luff in relation to the leech, so that you can get something that resembles a decent leading edge-raising and lowering, easing and squaring, are all methods to shape the luff properly for the angle and wind speed you are sailing-and allow the leech to have an open and clean exit (instead of being hooked inwards, backwinding the main, and tipping you over).
So much for the lesson. If you must use the sym kite this way, you can use the same techniques as you would with the A-sail-they just won't be as efficient. Slightly tighter tack line when sailing close to the wind, and ease it off the deeper you go. The tacker will prevent the sail from getting out away from the boat and into cleaner air where it can "breathe" better-especially as you sail deeper. In flat water, as one of you already mentioned, you can often sail ddw with the main on one side and the kite on the other-just remeber the kite will not be very stable without a pole, and it will be more stable with the tackline eased off and letting the sail get out and away from the boat about 5 feet or so (again, the tacker won't help here). If not, the luff will become very unstable and the sail will want to collapse at the slightest encouragement-there as many designs for sym kites (reachers, runners, AP sails, etc) as there are for A-sails, and how well it does for you will depend on what type of sym sail you have and what angle you are sailing.
Good night, and good luck!
S