I've done a little research on logging and have a few books. I don't recall seeing two log dumps on a single track separated like that, not to say it may not have been done. What I see here is a dual use of the log dump track. It is also used to load finished cut lumber from the mill. By using the track this way with only one log dump, a long incoming log train would be too long to be able to get the logs off of all the cars without encroaching on the finished loading process. I think Oakhurst did a good job of figuring out how to do it in a limited space. Very creative. However, the horizontal beams, poles, or whatever on top of or over the log dump equipment is a bit of a mystery to me as well.
OK, looking closer, the ends of the poles toward the pond could be cable attachments. The dumping cable would go from the pole ends, back over top of the logs on the car away from the pond, then under the logs on the car toward the pond, through a pulley and back under the track into the winch house. When the cable is pulled tight, it forces the logs off of the cars where they slide into the pond.
My stationary and winching points could be reversed. Maybe the cable was winched into the house from the rear of the boom, and a hook was fastened to the edge of the log dump ramp. That would make the ends of the boom toward the pond have pulleys for the cable. But each time a log car came up to the dump point, a man would have to take the cable over the top, and then under the log load. So yea, it now makes more sense that the stationary point of the cable is on the top of the log dump ramp.
Every time I look at Oakhurst's photo, I see something different. I have now noticed that he is using the same type of cars for his log loads as his outgoing cut lumber loads. So it is possible that the same cars that come from the woods with logs, could be shoved up to the cut lumber loading point after dumping logs.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.