The Treehouse - Working with Cables
There are a number of ways you can attach the non-bolted side of a supporting board to the second tree. You can bolt a J-bracket to the second tree such that the board can slide back and forth across it. But if you do this. you have to wrap metal around the wood part that slides through the bracket so that the wood doesn't wear out, then grease it so it doesn't squeak when it moves. J-Brackets can also be difficult to find.
Instead, you can do what I did and use steel cable. 3/8-inch plastic sheathed galvanized steel cable is rated to support 350 lbs. I used 2 separate lengths of it for each hanger to bring its support capability to 700 lbs. This way, the two corners it is hung from can each support almost the full weight of the structure on their own. Of course they won't need to, because the bolted corners can support more weight than the cable-hung corners. Overall, the structure's fasteners can support more than 4 times the weight of the structure. This means that no matter how many kids are up there, they'll never be enough to strain the supporting structure.
The picture shows the detail of the hanger assembly. The 5/8-inch bolt coming out of the tree gets a flat washer, then a nut, another flat washer, then a heavy duty cable tension adjuster (this allows the platform to be raised or lowered in infinite increments over a distance of about 2-3 inches in case the cables stretch a bit), then another flat washer, a lock washer (acts as a spring as the tensioner moves when the tree sways) then two nuts twisted against each other to lock them. The end of the bolt is filed to a bevel so there are no sharp edges.
At the other end of the tensioner, two separate cables are passed over the tentioner loop, using upside-down teardrop-shaped spreaders so the cables don't bend too sharply and crimp (that would weaken them). The end of each cable is laid back against itself, both sections running through a double-shaft crimpable coupling and fastened with a cable-crushing double nut U-shackle.
The other ends of each cable encircle the 2 x 12 support member and are fastened back upon themselves in the same manner that the other ends are fastened to the tensioner. (The second picture shows this) Either of these cables can carry the full weight they need to bear. The third cable, which crosses over the other two, is fastened directly around the other end of the bolt on the other side of the tree as a safety, just in case the tensioner or the bolt on this side should fail due to a manufacturing flaw or material defect. This cable is independent of the main assembly and bears no weight at all, unless the main assembly fails. This safety is also capable of carrying all the weight needed to support this corner. There are redundant, independent non-bearing safety cables like this on all four main support attachment points. Overdesign is a good thing when kids' safety is on the line.