SNAT Automap or SNAT pool for Outbound load balancing of 2 ISPs

In my experience, automap selects any available self IP. I think you'll need to use two separate SNAT pools. You might have to do it in an iRule, so when an LB decision is made, you apply the SNAT pool.

Not sure if this is an option, but if you have your own IP space, BGP session with the ISPs would probably work better for you (if you have dynamic routing on your BigIP and your ISPs are willing to do BGP with you).