Here's the quiz: How would you accomplish the placement of the second beam
onto the posts if you had to be self-reliant?
Knowing (now) the value of a back I'd have to give this a lot of thought.
My first thought would be to wait for help.
If impatience took hold I'd try hoisting one end up with a come-a-long, I'd
rest it on the top of a one of the ladders, then I'd do the same with the
other end. Now I'd just have to get the ends from the top of the ladders to
the top of the posts (or close).
Next I'd use my strong legs (thinking of my back), I'd raise one end enough
to get a piece of 2by under it, then I'd repeat at the other end. I'd keep
doing this or a variation until I was able to slide one end onto a post.
I'd be real careful because I'll bet that beam is heavy.
Good luck Gene!