Why does the Secondary Chain keep comming of?

I have one of these trikes and the chain issue is driving me up a wall also. First, make sure the rear sprocket is straight in line with the other sprocket. Second, you will find that the rear chain will loosen if the bolts are not tight, as the pull on the sprocket will allow the front chain tension to pull the front frame forward. I have also found that if I ride on a bumpy sidewalk, the rear frame will slip back, and the rear chain will be very tight. Obviously, this is because the bolts are not tight, but I have tightened these as tight as I feel that to avoid stripping something. My theory is that the two frames are flexing at the adjustment point and causing the nuts to work loose. I am going to strip the paint off under the nut area and use some Locktite. I am also considering adding some secondary hardware to lock the adjustment. I have noted that the clearence in the slots is a bit too much. This allows the joint to flex. If you could get some kind of shim or sleeve in there to take up that slack, I think that would fix it. If I find a permanent fix, I will repost it.

Any time a chain comes off it means there's slack in it or something's not tight enough. Not looking at your bike, I would have to guess that you need to loosen the bolts on the back wheels and then pull the wheel assemblies back until the chain is tight. This is probably a two person job, one to unbolt and bolt and one to apply tension. That being said, many years ago I remember bikes my friends used had a problem where if they were riding at a good speed and then stopped peddling to coast, then slightly reversed peddling, then started fast peddling again, that they would throw a chain. I think what was happening was they were creating slack in the chain by coasting, slightly reversing their peddling, then starting again. The chains were also slightly gummed up, allowing slack at this time.