I'm strongly in the camp that an advanced maneuver made from two base maneuvers is a single source.
1. In the section about combining maneuvers, is says to combine both effects. Not stack them.
Quote:
Effects: The effects of both maneuvers are combined. Single Target maneuver effects are applied to all Targets when combined with a multiple Target maneuver.
2. The advanced maneuver is a single maneuver that modifies your attack. Not two maneuvers. If it were two maneuvers then they would have to apply the stacking rules. (And would otherwise be illegal, but I digress.)
Take Rapid Strike as an example. Its effect is
Weapon -3 damage. If you were to combine Rapid Strike with Mighty Swing, you should end up with a Speed +0(5) maneuver that does
Weapon +2 damage. If you applied stacking rules while building an advanced maneuver you'd end up with a Speed +0(5) Mighty swing that still does
Weapon +5 damage. I don't think that was how Rapid Strike was intended to work.
A second (and possibly better) example: Mighty Throw and Careful Aim.
Mighty throw adds damage from making an exceptionally hard throw. Careful Aim adds damage from hitting a tender spot. When you combine them for an advanced maneuver, you should get the additional damage from both. Logically, that makes sense. (At least it does to me.) If you applied stacking rules when building this advanced maneuver, you'd only get the +5 from throwing it harder or from hitting a tender spot, but not both. You'd be better off using one of the two maneuvers instead of combining them.
I feel that the stacking rules apply to characters; not to Maneuvers. When you create an advanced maneuver, you're creating a single source that modifies an attack. That's why there are rules to create advanced maneuvers rather that just letting people perform two base maneuvers at once and applying the stacking rules.
Scott