A 510 is probably high enough but I find the problem is with the width, we have a 17.1 hh part shire and he won't travel in it as he can't get his balance. Would probably be ok on his own with the partition out but now just plain refuses to go near it. Think you may struggle to get 2 in if 1 is a Clydesdale, mine is 7 yrs old and I paid £3,000 for it.