There is a network device striping driver.  This works well when going from
linux box to another linux box.  This would properly balance your load under
this condition.  If your ISP was willing to so some funky routing on their end
you could use something like OSPF to try to balance the load.  One of the OSPF
metric is percent utilization  so you should be able to get them close.
How willing would the ISP be to work with you on load balancing your two circuits?

	Mike