I only have two horses right now. I plan to add more for the reasons stated above but in the meantime I have two. One I adopted last year and she is EXTREMELY herd bound.
Here’s what I have learned and some suggestions on things to try.
First, routine is key. I have gotten the herd bound horse to be okay with the other one going out to turnout first because we have now done that every morning for a year. But I am still in the habit of bringing them in together and when I try to bring them in separately, that is not okay. So everything is the same except time of day and it’s in reverse and that is NOT OKAY. (I plan to work on that.)
Second, high value distractions can really help. If I have both of them turned out on the dry lot and then I take my other horse to work her and I put my herd bound horse on the grass, that is mostly okay. Earlier this year when grass was a really big deal, it worked better than later in the year, and they were out on grass together during the day. Taking one away and leaving the herd bound one on the grass was not as successful.
Third, it’s easier to be the horse taken away than the horse left behind. I can take the herd bound horse for walks around the property and she’s fine as long as she is within eyeshot of my other horse. But she will tolerate a much greater distance when she’s the one out then the one left behind.
Fourth, consider drugs and/or a calming supplement. I had her on both a calming supplement and Trazodone for quite a while, which I think helped. She didn’t need a huge dose of the Trazodone, but it did help. I still have her on valerian and I suspect she’ll stay on that for the rest of her life. (She’s 28 so I do whatever I can to make her happy.)
Fifth, be prepared to accommodate the horse in ways you can’t imagine. For example, I leave a night light on for her. 
Sixth, try to enlist a helper. Sometimes I can get my husband to handwalk the herd bound horse while I work the other horse. That works well.
Lastly, be prepared with some Ace for times when the horse loses it and you’re worried about her. Better to have it and not use it than want it and not have it available. And make it injectable so you’re not trying to get pills in them.
Good luck! It is doable, but it is admittedly a pain. Once I finish my barn renovations, I am going horse shopping!