Ast cards should stack.

[Gemina]

Is this really an actual thing? Because there have been countless 24-man raids where I see one double healer party (or more) and looking at the other parties figure the algorithm should have just made a party with an AST/WHM each instead of AST/AST and WHM/WHM parties. Or AST/WHM and AST/SCH instead of AST/AST and WHM/SCH. Unless 24-man raids are the exception to this rule.

Indeed it is. Particularly in 8-man queues. For alliance raids, the algorithm is not perfect. Think LoTR when transporting locked out players are placed in the wrong alliance for the Atomos'. I've had those alliances as well where two of them have double of the same healer, but different from each other. Makes for some good lols. I actually don't mind it that much since I get to adlo the tank in the alliance without a shielder.