If your horses are confined to a stall or not moving hardly at all, their legs could just be stocked up (puffed up due to poor circulation, which is quite common, particularly in older horses) in which case, the best remedy is exercise (even if it's just hand walking for 15 or 20 minutes). I mention this, as that is typically the cause of swelling in both legs, particularly back legs, where no obvious cause for the swelling can be found.
However, if you feel heat at the swelling or your horses are lame, then this is not simply "stocking up" and I would recommend "standing their legs up"---that is wrap them with pillows/quilts/no-bows and standing wraps. Different people prefer different treatments under the wraps such as poultice, furazone, etc. but I might just start out with some liniment gel or mineral ice (which you can buy at any drug store). This treatment could also be used for "stocking up" but usually isn't neccesary. NOTE to never use liquid liniment, as this is meant to be diluted and put directly on a leg and wrapped could blister the skin. Lastly, if you are unsure how to wrap, be sure to ask someone to show you, as if done incorrectly, it could injure your horses' legs further. Additionally, if you don't own wraps and want a fix until you can get wraps, I might just put the liniment gel/ mineral ice on the legs. If your horses are lame, I would also administer 1 or 2 grams of bute twice daily for a few days and see if that helps the swelling/lameness.