I'm so very sorry about Lemby.
Don't make any decisions about Tango or another horse for a little while. Give yourself some time to grieve and then you may feel differently. I did--there was a time, right after I lost my favorite cat, our Himalayan, I vowed that I was never going to have any more cats--when our other cats were gone, that was it, I wasn't going to do that any more. Well, some time after than I was walking past this little pet store & there were free kittens in the window....I came home with one. Don't ask me how many cats I have now...I still break my heart over each & every one that passes, but I do love having them & they live good lives here. Horses too--it hurts to lose one, but I still have them.
Don't blame yourself. No matter what you did there may have been no help for Lemby. Sometimes there just isn't anything you can do. Perhaps it was a poison--maybe that branch of berries you have is what caused it--but something like that in the hay is so easy to miss and once the horse eats a lethal amount (which is not very much really) it's too late in some cases. It may not be that at all--though poison does seem likely since both horses were affected. But, sometimes some symptoms can point to a number of things. All you can do is guess & hope you give the right treatment, but when they fail so fast there simply isn't time for tests.
I'm just so, so sorry for your loss. I hope Tango feels better real soon.