I’d just start in his stall. It’s already a place of quiet and comfort, so it’s a great place to begin. Tie him, ride/work/groom the other one, come back, switch them. Tying is such a valuable and underrated tool. I think a lot of people have somehow contorted it in their brain that it’s cruel, when it absolutely is not.
I tie everything for at least 15-20 minutes after we get back from a ride. I call it their thinking time. It teaches them not to be in such a hurry to be done with the work, because they aren’t going to immediately get to cavort around with their buddies. We untack, hose off/cool off, and tie while I do a few chores.
Heck, think of the western barns. All the colts get saddled in the morning and stand tied at the rail until it’s their turn, and stand there until everyone is done.