I have this double canvas swag that tends to sag in the middle. What options do I have to fix this problem and keep the canvas from sagging too much?
At the risk of suggesting the obvious here:
1) Rig up an internal hoop. Use a spare of one of the external hoops and cut a few inches off to fit. Pad the ends, and Velcro the ends and top to the inside of the tent.
2) Put a few more guy-ropes on the end hoops to encourage some more lateral tension.
Second option should be the easiest one to try.
The main thing is that the outer guy-line needs to pull in a direction which pulls the top of the hoop outward effectively. Extending the guy-line in the picture farther away from the tent and closer to a horizontal angle is not an effective way to get more outward pull at the top of the hoop, surprisingly. It will pull the sides of the hoop somewhat, but to pull the top of the hoop away from the middle of the tent, the guy line should be perceived as an extension of the canvas eave. So, find the maximum angle which the eave can make away from the tent, and make sure that the guy line pulls toward the ground in that direction. So, you'll stake the guy line to the ground more closely to the tent. When pulling the guy line farther outward doesn't pull the eave farther outward, that's the point to stop looking for a stake point farther from the tent.
Once you find that angle, you'll want the guy line to be under as much tension as reasonable. Given the angle, you can see, I'm sure, how "pulling down (and out, but not too far out) on the guy line is the same as pulling up (and out) on the top of the hoop". So come up with a way to make the guy line tighter. You'll want to ensure that the stake or sandbag or whatever's holding the guy line down is as secure, heavy, or otherwise immobile as you can, and then use various means of tightening the tension on the guy line against that resistance. The tautline hitch knot or a plastic or metal guyline adjuster tab at the end of the line are options.
Last, you can also consider adding a guyline directly to the top of the hoop so that you CAN angle a guyline in the horizontal direction (which isn't optimal if the guyline is fixed to the canvas eave). Angling your guyline farther outward lets you take advantage of stakes which have less holding power, but only if it's attached to the top of the hoop and not to the eave. Once the line from the top of the hoop is fast and pulling outward (away from the center of the swag), then you still would use the other guy line, but only for holding the eave out, not for holding the hoop up.
With a shelter like this, it's mainly the ridge pole that tensions the fly.
You don't give details, but usually the length of the ridge pole will be adjustable. If the fly is sagging, try using a longer length to add more tension.
If the ridge pole isn't adjustable, you may have to modify it or source a longer pole.
Also, when pitching be sure the ends of the hoops are as far from each other as possible both lengthwise and side to side, so the groundsheet is fully stretched.