Hi Chris,
You will be able to have the reply traffic that comes in through one ISP go out via that same ISP. Let''s say that 192.168.1.12 is an IP in your 2nd ISP block and its public IP after translation should be 200.1.1.1. Please try adding the static NAT as follows for www traffic:
static (inside,outside) tcp 200.1.1.1 80 access-list isp2_allow
access-list isp2_allow permit tcp any host 192.168.1.12 eq 80
Or you can also use static PAT as an alternative:
static (inside,outside) tcp 200.1.1.1 80 192.168.1.12 80
This will translate all traffic destined to 192.168.1.12 and let the reply traffic out to the 200.1.1.0 subnet, which is the 2nd ISP subnet.
Hope this helps!
Regards,
Anu