To quote Optima's eCare Manager OptimaJim from mp3car.com, you can parallel Optima batteries.
If you read JMelton elsewhere on this site, people even parallel un-matched batteries "all the time".
Neither above provided evidence nor data to support their claims.
If you read people like me, we agree with the above and add that the batteries will fail quicker and when one fails, both do (unless you have complex monitoring etc).
This is supported by simple risk or reliability analysis. Also numerous reports by various authorities, and people's experiences - eg
The Official Dual Battery Thread (MasterCraft).
A few battery threads on this site put doubts against Optima's (recent?) quality. Others don't.
Locally is seems Optima rarely (if ever?) services warranty claims - the buyer has to prove it is NOT their installation's fault (ha ha bluddy ha - Deja Vu!). I have heard the same said of Odyssey.
For audio system, the second battery is usually used instead of a capacitor. Hence it is "next" to the amp, and will be of low ESR type and need not be a deep-cycle type.
If the battery is intended to provide added reserve time, then a bigger battery is used; probably a deep cycle type.
If the batteries are internal, they must be sealed (AGM or Gel) unless in a vented enclosure.
Whenever your alternator is unable to supply the current drawn by its loads, its output voltage will fall from (say) 13.8-14.4V, thru the battery(s) "surface charge" capacity to below the battery's fully-charged Open Circuit voltage of ~12.7V - ie, the battery will also supply the load.
The battery will eventually flatten if its discharge is not replaced (ie, recharged).
The time this takes depends on load, alternator & battery capacities.
Voltage drops from the alternator to the amp will reduce the voltage at the amp and its battery.
If the battery is less than fully charged, it will have a higher ESR (which may make capacitors seem more attractive if you believe their hype).
The lower the current, the lesser the voltage drop. (Hence the distant audio battery should fully recharge from a (say) 14.4V alternator given the chance to do so.)
Hopefully the above enable you to choose what suits your requirement.
FWIW:....
Isn't higher AH capacity & lower price desirable? Or is ESR important? If so, will that battery remain fully charged? If so, then reserve time is not important.
Systems can be set up to maintain 14.4V at the amp & battery.
Multiple parallel batteries can be switched by cascading relays from the first battery isolator.
If a charge lamp exists, then usually plain relays (contactors) can be used to isolate batteries cheaply.
If no charge lamp exists, then a less desirable or more complex & costly voltage sensing switch or battery isolator is required, but that can then control other cheaper relays for more batteries.
Isolation of all paralleled batteries overcomes any issues (perceived or otherwise) of whether batteries can be paralleled. It also enables non-matched batteries to be paralleled.
(Isolation in this context means "except when charging". It doesn't exclude "paralleling when required".)
PS - is it strange that people never thought to use the charge lamp circuit? F.ex - in the aforementioned
MasterCraft Official Dual Battery Thread, wouldn't that overcome many concerns?
Instead of connecting the relay to a switch or IGN or ACC, merely connect it to D+ or L of the alternator(?).