How to Access Other Gateway on Proxy server.

Hi

Issue is with my Proxy Server,

My Proxy Server has 2 Lan Cards.

eth1=10.5.1.100/24 dns:10.5.1.13 and No Gateway
eth0=10.0.0.10/24 and Gateway=10.0.0.2

eth1 is connected with switch which then connects to router and traffic goes all the way to our local application Server.

eth0 is directly connected with DSL modem.

I want to access my application server from This Proxy server. But my gateway of eth0(internet) takes and traffic because its the Default gateway.

I then have to disable ISA and provide default gateway to eth1 of my router to access application server but then My internet sharing stops.

How to Manage both of them?