Sneak attacking and Gestalt issues

[Eonir]

So I made a character for a duo gestalt campaign for a friend. He wanted a rogue with some spellcasting. So I made a rogue/spellthief. Now, I am aware that this is an un-optimized combination and is underpowered. All I need to know is does the SA from both classes stack, or overlap?

[FMArthur]

It doesn't stack when you get it at the same levels. If you skewed either side of it by a single level by dipping another class, all of the Sneak Attack would line up in a way that gets extra dice at different levels.

Anyway, Rogue/variation on Rogue doesn't make for a stellar gestalt character. Why don't you just do Rogue/spellcaster? That's pretty much exactly what was asked for. If you're set on Rogue/castery Rogue, you might as well take a look at Psychic Rogue (http://www.wizards.com/default.asp?x=dnd/psm/20040723b) as well. It's better than Spellthief and much simpler to play as well.

[Siosilvar]

It doesn't stack. But as Arthur said, why not just do Rogue//Sorcerer or something like that? Spellthief has wimpy casting and is basically the poor man's non-gestalt Rogue//Sorcerer anyway.

[Suddo]

Swashbuckler // Spellthief
Rogue // Wizard
Do that till the end. (Maybe a PrC or something too)

The two feats that fuel this are Master Spellthief and the Swashbuckler and rogue stack for stuff. This should make you only make you slightly SAD (Int > Dex > Con). If you can get Lesser Tiefling it will help a lot.

Oh and Psychic Rogue is pretty sweet. Though I don't know if it works with Swashbuckler but you should be able to do something like:
Psychic Rogue // Wizard
Psychic Rogue // Master Spellthief
Psychic Rogue // Wizard
That to the end, probably PrCing the Wizard side.

Edit: And I think they stack no matter what when you get them so. Its +1d6 not 1d6 Sneak Attack.