Hmmm...I am not sure what loose minerals you would have access to, given your area, but I am sure if you asked someone at your local feed store, they could tell you which one is good for your horses; I prefer 'horse specific' ones, as opposed to livestock ones, as I know that the horse mineral will/should cover what my horse needs specifically, instead of just a general livestock dosage.
Most of the loose minerals I have used, are designed for free choice feeding, although you can give a specific amount in their feed each day as well, as a general rule. I usually feed free choice. I use those hang over feeders, and make sure they are in a dry place, like a stall or run in shed; somewhere they have access to atleast a few hours a day. Since you have two, make sure there is a mineral supplement in a pen that they both have access too, again, atleast for a few hours each day.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."