The heating situation partly depends on what the temperature in your house is like. The UTH by itself is likely not enough to provide both the hot spot temperature and the right ambient temperature everywhere else, so it makes sense that you would need to supplement with the lamp. However, that's still not ideal because you presumably still need it at night. I'd replace the heat lamp with a ceramic heat bulb (you can use it in the same fixture that you already have), which doesn't produce light, so that you can maintain good ambient temperatures all night without having to have the light on. Another alternative is a radiant heat panel, or an additional UTH set to a lower temperature.

I wouldn't worry if the snakes don't switch hides during the day, that's like not getting up in the middle of the night to switch bedrooms. They settle in where they are, and then most of the time they stay put until it's wakeup time. The other thing that is helpful is providing stuff like fake (or real) vines/plants, tubes, etc, so that the snake can get from one side to the other with some cover. It might not be so much a preference for one hide or another that prevents a BP from switching, as a preference not to cross open ground.

For what it's worth, the snake in the warm hide was probably not sitting there at 96 degrees; if the UTH was set to that, the temperature on top of the substrate is probably a lot lower. If you have an IR thermometer, you can point it at the snake to check its body temp; I'm sure it isn't 96 degrees. The snake in the cool hide might not have been all the way down to 70, either; if the hide is a dark color it could still be absorbing heat from the lamp and get warmer inside than the ambient temperature around it.

That said, 70 is pretty low for a BP. You should shoot for high-70's to 80 for the cool side and 90-ish for the warm side. It doesn't matter that much how you get there.