It does sound like you are looking for ECMP from the description. This configuration allows both ISP in the same routing instance and balances on a per flow basis for outbound traffic. Note that for this to work for your inbound destination nat traffic you will also need to perform source nat on the interface for that inbound traffic in addition to the destination nat. This willl insure the reply traffic maintains that flow and does not use ecmp to egress on the other ISP.
Another options is using FBF (filter based forwarding) that allows you to steer traffic by source ip address or port to use one ISP or the other to share the traffic load. These are examples of this approach.
I am not sure I follow your description of the flows. Sorry if these are not what you are seeing.
For the outbound traffic remember that ECMP binds flows to the same next hop. So as a general rule traffic from the same ip address will use the same next hop.
It also seems like you want inbound nat traffic to be sure to return to the same ISP. In this case I generally recommend having the traffic use source nat to the srx interface in addition to destination nat. This makes the srx interface itself the return address of the packet and forces the flow out the desired interface.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home