hi,
you can do the ff:
-create 2 routing-instances type virtual router for your 2 isps
-create 2 security zones for your isps
-assign 2 interfaces for your isps
-bind those interfaces to your virtual routers
-i would suggest to use an L2 switch and individual ports (if your using srx 240, you got lots of ports)
-your trust network will remain in the inet.0 routing instance (trust-vr in screenos)
-configure static route with next-hop on each routing instance-virtual router
-then define rib groups which will allow route sharing of inet.0, isp1.inet.0 and isp2.inet.0 instances
-define static nat trust -to-isp1
-define static nat trust-to-isp2
-configure proxy-arp for those 2 isps interfaces
-create policies
there you go.
for additional information, you can check juniper website on routing-instance and rib-groups.
//dwayne