I have the rapid reload feat for the Wrist Launcher, Heavy weapon and since you can wear one of these weapons on each forearm and functions much like a hand crossbow, that free loading both would be easy both hand are free, unless holding something.
However I'd like to be able to you use both in combat together, but confused as how to do it. Do I use it with Rapid Shot or Two Weapon Fighting?
You can use all applicable abilities including both two-weapon fighting and rapid shot. (See this question for a bit more in depth detail about Two-Weapon Fighting.)
More importantly though:
Under Wrist Launchers, we see:
If you are proficient with hand crossbows, you are also proficient with wrist launchers.
This does not mean they act like hand crossbows for how things like Rapid Reload affect them. Instead, it merely means that you're considered proficient with them if you're proficient with hand crossbows. This makes even more sense if you consider the fact that a hand crossbow takes a move action to reload, and a full round action for a Wrist Launcher.
An example of a weapon that copies another weapon is the Aldori Dueling Sword:
An Aldori dueling sword can be used as a martial weapon (in which case it functions as a longsword)
Note that the Aldori Dueling Sword actually functions as a longsword in this case, unlike wrist launchers which merely get proficiency.
Another example is the Dwarven Pelletbow:
A dwarven heavy pelletbow takes the same amount of time and effort to load as a light crossbow (normally a move action)
Unlike the Wrist Launcher, the pelletbow calls out that it uses the same load time as another weapon.
Choose a type of crossbow (hand, light, heavy) or a single type of one-handed or two-handed firearm that you are proficient with. You can reload such a weapon quickly.
Note that nowhere in the text of the item is the wrist launcher called out as a type of crossbow, nor is it even in the Crossbows Weapon Group like the pellet bow or tube shooter. The relationship with Hand Crossbows is the only thing linking it with crossbows and allowing it to be selected by the feat.
If we look at the Tube Arrow Shooter, it says:
Reloading a tube is a full-round action, or a standard action if the wielder has the Rapid Reload feat.
Unlike the Wrist Launcher it specifically calls out that rapid reload has an effect on it (and thus implying you can take rapid reload for it).
If you are using two wrist launchers (or two hand crossbows or two one-handed firearms or two thrown weapons or any other pair of ranged weapons), you can take and use both Rapid Shot and Two-Weapon Fighting. The penalties stack, but you also get more attacks.
This requires being able to reload the weapon fast enough. As william porter’s answer describes, it’s not clear that wrist tubes can even benefit from Rapid Reload in the first place—and if it does, it most likely makes it a standard action to reload the wrist tube. That’s too slow to be useful: Rapid Shot won’t matter because you won’t have a loaded wrist tube to fire. A wrist tube probably doesn’t even let you draw another, preloaded wrist tube with Quick Draw, which would be plausible with some other weapons (though even there the costs get to be much too high).
The long and short of it is that wrist tubes are bad weapons; at best, you get one attack per fight out of one, which doesn’t seem worth the effort of bothering to have one. If you want to dual-wield projectile weapons, use hand crossbows. Then you can use Rapid Reload, Rapid Shot, and Two-Weapon Fighting to get two extra attacks (and a −4 penalty on all attacks). Improved and Greater Two-Weapon Fighting add more attacks. This is a lot of feat, and rather hefty attack penalties, but at least it actually works.
