Think Brazen Atk/Res seal will stack with same A skill?

Sadly...

Yes it stacks since its a "in combat" buff

Yes. This already happens with Alfonse's refined weapon (which has Brazen Atk/Def) or Celica's refined weapon (Brazen Atk/Spd) and any Brazen (or in-combat skill, really) in the A-Slot.

wait...this with alfonse...

Folkvangr (Res)
Ardent Sacrifice
Miracle (For memes)
Brazen Def/Res 3
Vantage
Flexible C slot
Brazen Att/Res 3 Seal

WELP, DAT +14/14/-/14

... holy crap. My Caeda is going to love that. I'm just realizing. Her A skill is Brazen Atk/Spd. Combine that with Brazen Atk/Res and all of her important stats are buffed as fuck.

+7 merge with Summoner Support
50 base Atk. +14 = 64 Atk in combat
42 base Speed. +7 = 49 Speed in combat
39 base Res. +7 = 44 Res in combat
All with Flashing Wing Sword, Desperation, and Aether. With flying buffs.

Bless this new seal. This seal will make my Caeda even more op. Your Armors better watch out for her in Arena.