A Guide to Ability Learning

Found it:

In the middle of the learning code for both Terra's and Celes' magic there are addresses for the learned magic list to write to:

Terra's:
C0/A1AC: 6E 1A ($1A6E)

Celes':
C0/A1CE: B2 1B ($1BB2)

These need to be changed in order for the correct actor to learn the correct magic when the character is created in-game.

Actor - Spell List
$00 - $1A6E
$01 - $1AA4
$02 - $1ADA
$03 - $1B10
$04 - $1B46
$05 - $1B7C
$06 - $1BB2
$07 - $1BE8
$08 - $1C1E
$09 - $1C54
$0A - $1C8A
$0B - $1CC0

Each is essentially $36 higher than the one before it. I haven't tested all of these addresses yet, but I know 00, 01, 02, and 06 work, I am assuming the convention stays the same throughout.

One neat little note: if you drop yourself somewhere in the middle of those values, your learned spells are going to be offset by the same amount. When I set Terra's magic to write to $1AA5, instead of learning Cure and Fire, actor 1 learned Cure 2 and Ice.