It's ordinary block and tackle mechanics. Using two pulleys in place of the TH loop and the object that the line slides around would give a mechanical advantage of 2 - that's the most it could be. Technically there would be a reduction for friction, but commonly it would be called 2. IIRC, in seamanship, the configuration would approximate "gun tackle."

Line sliding over line and around things like trees, posts or rails would seem to significantly reduce the mechanical advantage due to significant friction. I have no empirical evidence, just common sense that says it would be significantly less than 2:1.

I'd guess that the internet has lots of resources explaining rigging or block and tackle mechanics if you'd like to learn about it. Just stay away from the sources that say it's 3:1