THEORY: Mixed erg/acc

I have a question about langs that use ergative/absolutive
for all sentences, except for 1st and 2nd person, which
use nominative/accusative (e. g. Dyirbal and my G'amah).

Does this mean that

a) the 1st and 2nd person pronouns are marked NOM or ACC
   while the other argument is marked ABS or ERG, respectively,

OR

b) when the subject is 1st and 2nd person, it's marked NOM
   while the object is ACC?

(Other alternatives, maybe?) Any answer, or at least ideas
on likeliness of either kind of syntax, would be helpful.
I don't want to go "D'oh" once I finish writing my magnum
opus on G'amah and find out I got it all wrong...


--Pablo Flores
  http://www.geocities.com/pablo-david/index.html
  http://www.geocities.com/pablo-david/draseleq.html