Ditto what Peggy-Sue said.
Also, have they been de-wormed? Worms and tooth issues could also add to why they aren't gaining weight.
If it hasn't been done yet, I would have a vet evaluate both of them including checking their teeth and giving him or her a stool sample from each horse.
They could also have encysted strongyles, which would mean worming them with a Power-Pak ---- again something the vet should evaluate.
Finally, if each horse is down ~200 pounds, it is going to take them a long time to get back up to speed. They aren't going to look their "A" game overnight, so try not to get upset by any outside comments
When I rescued my Arab 15+ years ago, he was a head attached to a skeleton.
He was so down, he got fed a little bit 3X/day instead of the normal 2X/day everyone else got fed.
While he started making immediate progress, the progress curve would sometimes be flat, then it would spike, then be flat again for awhile.
It took him until the following year to look like the show horse that I had no intention of making him be
I have rescued 3 or 4 starving horses over my life. My personal opinion on horses that are even 100 pounds underweight is to not rush them into "fatness". They need to be brought up to speed at a slow but steady rate.
Hope this helps you some