I don't believe Tulsa gives you a choice. I think you have to sell the one that ends up highest in sale order - it's been that way forever, unless it's changed recently. If you have a pretty good idea of which one you want to take to Kansas City before you nominate, always put that one in a breed that shows later - then if your other steer makes the sale, you can scratch the second one and just not show him. At OYE you can move from any breed to cross when you turn in your cards - I don't know if Tulsa allows this or not. I doubt it, but it's worth checking into. At this point there's not a whole lot else you can do.
Edit to add; Just checked the rules - It does say exhibitors must sell their highest ranking animal in sale order.