As for exercising both, you could learn to pony the second horse and you could switch at your halfway point and ride the ponied horse and pony the riding horse back. Sort of confusing but you probably get the point.
I have two riding mules, one close to retirement and the other with 60 days training (she is six), I ponied the green mule many miles on trails, and roads exposing her to wooden bridges, water crossings, railroad tracks (and trains gulp). She has now been on two camping trail rides (3 days each) and has been a gem.
Oh, and while ponying her, I filled her saddle bags with empty pop cans, hung crinkly slickers off of her and dangled my chinks and what ever else I could find to desensitize her. Best decision I ever made was to do all of this.
Wendy B / Sedgwick, KS / Extreme Trail Riding / Camping