I would imagine you're pretty unlikely to find one - most 3.5 tonne boxes I've seen have space for 2 ponies (Probably not the weight for 2 horses) and a bit of storage space for hay, water, tack etc, possibly a bench seat to fit 2 people + driver, but I wouldn't have thought more than that.
You could get a 3.5 ton chassis pan cab with a crew cab but it will be heavier than the normal chasis cab and you will probably find you will only be able to take one horse to avoid going over weight.
Northern horseboxes will add in extra seats. I have a friend who has had this done. Instead of having the tack space at the back, 2 fold away seats have been put in (with seatbelts), for her kids to travel in/on.
I would not travel a horse in at the same time though. I don't think it's safe.
the biggest of my two is only just 11 hands & i only carry bridle & grooming kit each plus a barrel of water & living isnt really anything i bother with but being able to get us out & about is.