If the second modem/router is connected to the Lan switch you are not protected by the OBSD firewall at all.
Insert a third NIC on the OBSD firewall and connect the second modem/router to it.
See
http://openbsd.org/faq/pf/pools.html#outgoing for a pf ruleset wich will do load balancing between two internet connections.
If some servers use the first ADSL connection and some the second one, the ruleset could be less complicated.
The most secure and recommended solution is to put all the servers in a so-called DMZ. That would require a fourth NIC though
