One thing I’ve done is to have the daytime hay in the extremely slow feeder bags (for instance, the Shire Greedy Feeder ones). It slowed the fast eater (who didn’t need the calories) way down, without slowing the slower eater (not a hard keeper, just more normal) as much as one would think.
Then I’d make up the difference needed with the night bags.
On pasture, I’d muzzle the one who didn’t need so much grass, but I don’t like to leave a muzzle on for more than a few to several hours. Normal grazing here is never more than four or five hours per day (sometimes broken up into two sessions), due to our climate and available grass, with horses mostly kept in a large sacrifice paddock with hay.
Much as I like the idea of keeping them out 24/7, and I’ve done that as much as possible, I’ve found it works well to have them up overnight (eight or nine hours), which allows the slower/needs more horse(s) to catch up on calories. I’m pretty quick at picking a stall, so really not much extra work or expense (I’d be picking up the paddock anyway, or picking and/or dragging the pastures).