alright, so here is the thing based on my experience:
1. tacacs for shell/gaia/clish is one thing, tacacs for Management (SC) is another, bear that in mind please
2. failover with ACS is another story and I believe it has nothing to do with Gaia (you need 2 acs servers configured for redundancy of AAA do you?)
3. gaia and other systems like that will always see AAA server as one but pool both so you should be comfortable having them both configured and both "pool' able" however, as far as I know the resource used for AAA will remain one-at-the-time for users.
try experimenting and first see if both "allowed" ACS servers talk to the GAIA itself and SMS at the same time (fw monitor?) and see if the tokens are issued correctly (logs?). then try to differenciate users between shell (gaia) and SmartConsole (Management/OPSEC) setup.
let me know how it goes, I do have similar setup with RADIUS with one of my clients but it shouldn't be that different really ...