Buy the friggin' ladder stand! It is much easier to put up and is mobile. Permanent are just that permanent. Plus no matter what materials you use, they will only last two to three seasons before the wind and squirrels have them in need of repair.
If you must build the permanent, don't put it in multiple trees. During a decent wind event, each tree will sway in different directions at differing degrees. They will tear the stand apart. When I used to build them, I would find a tree with a "Y" and place the boards across the "Y" to create the platform.