Do attack bonuses from Scurrying Swarmer and Underfoot stack?

[William Nova]

The way I read it, if I enter an opponent's square without an adjacent ally I can invoke Underfoot, but if there is an ally adjacent to my opponent I can invoke both Underfoot and Scurrying Swarmer.

One bonus is Flanking, the other is what I think folks call Untyped, so to me these to-hit bonuses would stack. I'm new to PF, so I'm not sure if this is how it works or not.

Dirty Tactics Toolbox wrote:

Scurrying Swarmer (Combat)

You abandon all notion of personal space when in danger, scurrying about your allies in search of the best vantage point from which to strike at your foes.
Prerequisites: Ratfolk, swarming racial trait.
Benefit: You can use your swarming racial trait to enter the square of any willing ally that is the same size as you or up to one size category larger, granting you the benefits of the swarming racial trait as if your ally were also a ratfolk with the swarming racial trait. Your ally gains no benefits from sharing its space with you unless it also has the swarming racial trait. Additionally, you treat any ally who shares its space with you as having the same teamwork feats that you do for the purpose of determining whether you gain a bonus from teamwork feats. Your ally doesn’t gain any of the bonuses from these feats unless she actually has the teamwork feats. You and your ally’s positioning and actions must still meet the prerequisites listed in a teamwork feat’s description in order to gain the teamwork feat’s listed bonus, but your ally is considered to be adjacent to you while you share her space for the purpose of determining whether you or your ally gains a benefit.

ARG wrote:

If two ratfolk in the same square attack the same foe, they are considered to be flanking that foe as if they were in two opposite squares.

Blood of the Beast wrote:

Underfoot (Combat)

You clamber in and around your enemies’ personal space to devastating effect.
Prerequisites: Dodge, swarming racial trait.
Benefit: You are able to share a space with a hostile creature at least one size category larger than you. You gain a +2 shield bonus to your AC while occupying an opponent’s space, as well as a +2 bonus on all attack rolls against the creature whose space you share. Creatures you share a square with take a –2 penalty on all concentration checks. Entering a space occupied by an opponent provokes an attack of opportunity. This ability does not stack with similar abilities providing a creature benefits for being in another creature’s space, such as a mouser ACG swashbuckler’s underfoot attack deed.
Special: This feat counts as the Disruptive feat for the purpose of qualifying for other feats that have Disruptive as a prerequisite.[/quote

[Taja the Barbarian]

Scurrying Swarmer requires you to be in the same square as an ally while Underfoot requires you to be in the same square as a hostile creature: The only way these two can be combined is if you and that ally are in the same square as the creature you are attacking (your ally is another ratfolk with the Underfoot feat or is a Mouser Swashbuckler).

If you and an ally are in the same square as your target (which is an incredibly unusual situation unless you are both specifically building toward this), then the two feats should probably stack, but your GM has the final call on this...

[Derklord]

The question doesn't really make sense - Scurrying Swarmer doesn't grant a bonus. So the actual question is whether Underfoot stacks with the bonus from flanking, which is a definite "yes":

"Bonuses without a type always stack, unless they are from the same source." CRB pg. 208

Here's the whole section, as you might have not read it yet, seeing as it's in the magic rules:

Bonus Types: Usually, a bonus has a type that indicates how the spell grants the bonus. The important aspect of bonus types is that two bonuses of the same type don’t generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works (see Combining Magical Effects). The same principle applies to penalties—a character taking two or more penalties of the same type applies only the worst one, although most penalties have no type and thus always stack. Bonuses without a type always stack, unless they are from the same source.

[Taja the Barbarian]

Derklord wrote:

The question doesn't really make sense - Scurrying Swarmer doesn't grant a bonus. So the actual question is whether Underfoot stacks with the bonus from flanking, which is a definite "yes":

"Bonuses without a type always stack, unless they are from the same source." CRB pg. 208

** spoiler omitted **

In theory, they should stack, but it's going to be very difficult to use both abilities at the same time:

• Generally speaking, you can not be in the same square as another creature unless all but one of you are incapacitated.
• Swarming trait specifically allows two ratfolk (and only two) to share a square and grants them the Flanking bonus.
• Scurrying Swarmer allows you to treat a non-ratfolk ally as a ratfolk for the Swarming trait, allowing you to share a square with one ally (and apply teamwork feats your ally doesn't have).
• Underfoot (BotB) allows you to enter your opponent's square and get a bonus to hit and AC. However, once you do this, you are no longer sharing a square with an ally and therefore no longer gain the 'flanking' bonus from 'Swarming'. (Unless otherwise specified, 'flanking' requires you to be on 'opposite' flanks, so a character in the same square as the target can not gain or help anyone else gain a flanking bonus).
• If your ally also has the ability to enter you opponent's square and does so, then we are in 'Table Variation' territory: Swarming has a '2 Ratfolk in a square' limit which sorta implies you can't fit two ratfolk plus an opponent but this sort of ability interaction is not spelled out anywhere so your GM has to decide for how these abilities combine at your table. (There are teamwork feat that increase the 'two ratfolk' limit, but they still do not cover the 'plus one hostile target' complication)

So,

if you are a ratfolk with Scurrying Swarmer and Underfoot (BotB) feats,

your ally is either a Mouser Swashbuckler or another ratfolk with the Underfoot (BotB) feat,

Your GM decides you both can occupy the same square as your target at the same time,

and you do so,
THEN you would get both the Flanking bonus and the Underfoot bonus.

[Derklord]

Other methods of flanking, e.g. from Gang Up, Pack Flanking, or Blades Above and Below, would work, too - Scurrying Swarmer isn't actually a relevant part here. If you use Underfoot, and you somehow count as flanking the target, the two bonuses stack.