natd -p 8668 -a ip1 
natd -p 8669 -a ip2 

ipfw add divert 8668 ip from any to ip1 recv if1 
ipfw add divert 8669 ip from any to ip2 recv if2 
ipfw add check-state 
ipfw add prob 0.5 divert 8668 ip from 192.168.0.0/16 to any xmit if1 keep-state 
ipfw add divert 8669 ip from 192.168.0.0/16 to any xmit if1 keep-state 
ipfw add fwd gw1 ip from ip1 to any out xmit if1 (если default на if1) 
ipfw add fwd gw2 ip from ip2 to any out xmit if1