We have an ASA going to a router that is connected to two ISP's, but no BGP. The ASA is using PAT with an IP from ISP-1, so even when traffic is routed out to ISP-2, it comes back via ISP-1. Is it okay to do PAT again on the ISP-2 interface, so traffic will come back to this interface?
The voice of experience says it should work ok. I have a customer where we have a very similar situation. The customer traffic passes through a firewall where the addresses are translated using address space from the primary service provider and forwarded to the router with the connections to a couple of service providers. If the traffic is to be forwarded to the second provider then we translate it again. This is working fine for us.