If you have a slant, you put the first horse in, turn them around and back them into the slant stall. Close the divider. That way they can put their head over the divider and travel facing backwards. There's a slight risk- they could try to jump over the divider, but that risk occurs with straight load trailers too.
Or you give them the entire trailer and leave them untied. Most of my horses travel untied quite nicely.
But it depends on the horse. I no longer tie my horses in the trailer because if they should slip, i don't want them to hurt themselves. All have adapted to this well. Some travel just like they are tied. Others like to travel facing backwards.
If I'm trailering a new horse, i leave them tied.