I think your chances of getting a 15.1 on a transit herringbone are nil - they're just not wide enough. Herringbone doesn't spread the weight as much as you'd think either as the hind feet are much closer to the edge of the box than the front feet, which still puts more weight on one side of the box.
Transits tend to work on forward facing travel rather than rear facing. Not sure if there's a specific weight distribution reason for that though. The heaviest horse always goes behind the driver as that compensates somewhat for the camber of the road.
If you're thinking of travelling both horses you've put in this thread though, check your weight limits. I'd certainly not put that combination in my 3.5 tonne box as it would almost certainly take it over weight limit. You need to check both the total weight and axel weight, especially where the horses are over the rear axel as they would be on a transit base.