i suspect that all proxy traffic is going through the ISP1 that is set as default gateway and/or is first listed as default gw (destination = 0.0.0.0/0)
IMO if you want to divide traffic that is aggregating from your router you have to load-ballance your traffic on outgoing chain for dst-port=80
that way you may achieve that proxy is working through 2 gateways.
as far as i know that is not that simple if your proxy is on the same machine where load-ballancer reside