Ast cards should stack.

[Seraphor]

Something I posted in the AST 5.0 Wishlist topic, would seem to apply here.

New Spire effect: Reduces MP costs for target by 10%.
This would compliment it's counterpart in Ewer, while Ewer would recover lost MP, Spire would preserve MP, and with TP merging into MP this will benefit anyone.

Sects effect cards: Current effects are present in Diurnal Sect*, new effects for Nocturnal Sect target enemies instead of party.
Balance: Enemy deals 8% less damage for 15s.
Bole: Enemy takes 8% more damage for 15s. (does not stack with Trick attack)
Arrow: Enemy’s attack speed is reduced by 10% for 15s. (but certain special moves and phase changes are unaffected)
Spear: Enemy cannot deal critical hits for 15s.
Ewer: Inflicts a 50 potency DoT for 15s.
Spire: Enemy takes 10% of the damage they inflict on any party members for 15s.

The point of this is to allow dual-AST compositions to use their cards without worrying about overwriting the other’s effects.
The duration is nerfed to 15s for Nocturnal effects as they could be more OP than party buffs, with Bole/Balance nerfed in potency to 8% as well.
*Diurnal Balance and Bole also nerfed to 8% effect.

I think this would also see Nocturnal Sect favoured more over Diurnal in some ways, due to an Enhanced Bole being more effective than Trick attack. (12% more damage over 15s) Would be a neat way of improving the currently inferior Nocturnal Sect.

Some additional ideas for Noct Sect.

Nocturnal Sect Minor Arcana:
Queen of Coins: Bestows a HoT on target party member for 21s.
King of Coins: Inflicts a DoT on enemy for 21s.

Celestial Opposition additional effect: Extends debuffs inflicted by yourself by 5s.
Time Dilation additional effect: Resets the value of HP-shields.
(e.g. if a 3000 HP shield is erected, they suffer 2000 dmg, then TD would reset that back to 3000, but will have no effect if the shield is already fully depleted. Hopefully the indications that changes to the battle system UI will display HP shields will make this easier to manage.)

These would give Nocturnal Sect an extra HoT it can use but on RNG, and a way to modify its shields through TD.
I thought this was the most effective way to have you enhance the effect of Noct shields, but would be too OP to have two skills like this, likewise two skills extending the duration of debuffs would be OP if you could essentially stretch a Trick attack out to 25s.

I think this is markedly less overpowered than simply letting two Balances stack. Not only because my idea nerfs them down to 8% instead of 10%, but because the debuff wouldn't stack with Trick attack, meaning that meta party compositions would likely have to choose between dual AST and a NIN, and there will probably be other downsides to having Dual AST. But importantly when pugging, those Dual AST parties would benefit, but not be any more OP than if they happened to have a NIN with them.