Well, in the MM it says a heavy horse can drag 4,500 Lb (which fits with the PHB saying you can generally drag 5 times your max load - for a heavy horse that's 900 Lb).
The PHB goes on to say that favorable conditions can as much as double that number, and poor ones can halve it.
So for two horses in average conditions (normal wagon, roads, and whatnot) I would say 9,000 Lb. If it's a great wagon, perfect roads, and everything else is right, maybe as much as 18,000Lb - though these conditions are rarely met. On the other hand, a bad road, an old wagon, or something else could make it more like 4,500Lb.
So let's make a reality check. First, what IS a heavy horse? Well, according to the PBH, it's a Clydesdale.
So, I found a Clydesdale FAQ on the web (
http://clydesusa.com/clydesdale_faq.htm) …
http://clydesusa.com/clydesdale_faq.htm said:
How much can a Clydesdale horse pull?
It depends on what their pulling. When pulling a wagon, a Clydesdale can pull many times more than its own weight, which is why the Clydesdale was so popular in the cities pulling large wagons of goods.
How much does a Clydesdale weigh?
Mature Clydesdales weigh between 1600 and 2400 pounds, as much as a Volkswagon Beetle.
…and another Clydesdale FAQ (
http://www.diegelberosecarriage.net/id17.html)
http://www.diegelberosecarriage.net/id17.html said:
Is it hard for them to pull the carriage?
Clydesdales can pull 5-6 times their own weight on wheels. Considering that our carriages, fully loaded with passengers, weigh about the same as the horses themselves do- pulling a carriage is literally a walk in the park! More convincing is that a carriage is hitched to the horse- so if one of our 100 lb drivers can pull an empty carriage out of our carriagehouse, a 1800 lb Clydesdale can certainly pull a loaded one!
So, 5 to 6 times 1,600 to 2,400 sounds like it's in the neighborhood of 9,000. Seems OK to me.
The real question is going to be cubic feet of space. 9,000 Lb is a lot of stuff.
-Tatsu