You could also put a loft above and fill it with hay(will act as some insulation?). If two horses can be together they can stand together and use each others body heat. Do what you can to help them keep warm. Maybe a insulated floor?

Brad, a little info on horses. The are not people, we like to be extra comfortable and go to great extents to do so. Horses can actually get over heated if kept too warm. Mostly they just need to get out of the wind. They grow a winter coat just like all other animals. As you drive the country side you will see horses with blankets on. This is an attempt by the show driven owners to keep the animal in a summer coat, not the long hairy winter coat. In my opinion they are doing the horse an disservice. Not that they like to be cold, picture them standing broad side to the welcome morning sun.

More important than an insulated floor would be a dry floor out of the wind.