Do these extend spell effects stack?

[Voidstar01]

Argent Savant has this line:

Enduring Force (Ex): Force spells cast by an argent savant of 3rd level or higher linger longer than normal, and they are more difficult to counter or dispel. The duration of a force spell cast by the savant is doubled, as if the Extend Spell feat had been applied to it. However, the spell’s level and casting time are unchanged. The same restrictions that apply to extended spells apply to spells affected by enduring force. In addition, the DC required to counter or dispel the argent savant’s force spells increases by 4.]

Abjurant Champion has this:Extended Abjuration (Su): You depend on your abjuration spells to protect you in combat. Double the duration of abjuration spells you cast, as if you had applied the Extend Spell feat to them (but without any change in level or casting time).]

So assuming I do spell research to get a spell thats identical to mage armor but is an abjuration spell, would these stack to give me 4x duration? or would I only get double? what if I applied the actual extend spell metamagic to it?

[Oberron]

They don't stack, they both say as if extend spell mm was applied. You can't use the same metamagic multiple times on a spell unless otherwise stated.

[Voidstar01]

They don't stack, they both say as if extend spell mm was applied. You can't use the same metamagic multiple times on a spell unless otherwise stated.

Thank you thats what I thought but I wasn't sure.

So related question for all the DM's on here, would you allow the effects to stack? why or why not?

[JNAProductions]

Thank you thats what I thought but I wasn't sure.

So related question for all the DM's on here, would you allow the effects to stack? why or why not?

Probably not. That's a buff to casters, and they're more than strong enough already.

[Asmotherion]

i'd allow it. At the level you have both you can probably persist it reliably so you're not exactly toying with game balance.

Also if you get both PrCs you're devoting permanent resources (class levels) for an effect (that's not exactly OP just thematic and probably following designer's intention); i'd feel bad to tell you "hey there's this part in RAW that gets your character concept completely irrelevant". That would just encourage you to optimise more extreamly and that's how you end up with Deity-powerlevel Wizards.

[Biggus]

Thaumaturgist has this line:


Extended Summoning

At 3rd level and higher, all spells from the summoning subschool that the thaumaturgist casts have their durations doubled, as if the Extend Spell feat had been applied to them. The levels of the summoning spells don’t change, however. This ability stacks with the effect of the Extend Spell feat, which does change the spell’s level.

If you had two classes which said their extend ability stacks with Extend Spell, there would be a reasonable case to be made that they would stack with each other, but neither of the classes you mention say that.

I would be wary of allowing multiple extend abilities to stack, it makes it too easy for mages to get buffs which last all day, and as JNA says they're strong enough already.

[DwarvenWarCorgi]

At the very least, if your dm did allow them to stack, the 3.5 multiplication rules say that you dont double then double, you increase the multiplier. So double and double becomes a single triple.

[RNightstalker]

At the very least, if your dm did allow them to stack, the 3.5 multiplication rules say that you dont double then double, you increase the multiplier. So double and double becomes a single triple.

I would imagine time is a real world value that regular math applies to.

[Clementx]

I would imagine time is a real world value that regular math applies to.
You aren't multiplying time. You are multiplying duration. Double Extend is x3.