Due to the physical line limitations we can't have more than 2 Mb/s on
one DSL line. The DSL line is connected to hardware router with modem

My idea is to have second line with another hardware router connected to
another public network card in server which serves internet with proxy
cache made with BorderManager.

Is it possible to set up it to split traffic to/from Internet between
two public interfaces and two different internet connections? How can I
do this?

BorderManager 3.8
Novell SBS 6.5