I was looking over the weapons lists and found the Hand Crossbow among the options. Thinking of the possibilities, I looked over the Two-Weapon fighting feats and a number of them sounded like they were intended for melee weapons. Can someone explain how Two-Weapon Rend and Double Slice apply to ranged weapons?
The feat Double Slice allows the attacker to add his full Strength modifier to the damage of an appropriate off-hand weapon even at range
The benefit and normal entries for the feat Double Slice say
Benefit: Add your Strength bonus to damage rolls made with your off-hand weapon.
Normal: You normally add only half of your Strength modifier to damage rolls made with a weapon wielded in your off-hand.
An appropriate off-hand weapon is either a thrown weapon or a specially built composite longbow, specially built composite shortbow, or sling:
A character gets no Strength bonus on damage rolls with a projectile weapon unless it's a specially built composite shortbow or longbow, or a sling. If the character has a penalty for low Strength, apply it to damage rolls when he uses a bow or a sling. (Buried on the Weapons page; do a search.)
Let me make it clear that the above is my reading of the feat, based on the feat's normal entry, and, really, sometimes a feat's normal entry is the worst thing about a feat. So it's possible a more generous GM may allow an attacker to add his Strength bonus to the damage dealt with his off-hand light crossbow (or pistol or bola) while receiving no Strength bonus to damage with his primary hand light crossbow, but—and I'm gonna go out on a limb here—I suspect such a GM will be in the minority.
The benefit of the feat Two-weapon Rend when used with ranged weapons will depend on the GM
The benefit of the feat Two-weapon Rend says
If you hit an opponent with both your primary hand and your off-hand weapon, you deal an additional 1d10 points of damage plus 1-1/2 times your Strength modifier. You can only deal this additional damage once each round.
So, yeah, that's actually pretty okay with a pair of thrown weapons that deal damage plus (either whole or in part) the attacker's Strength bonus like, for example, daggers or—I don't know—spears or something, but it gets a little weird when going all The Killer with a pair of hand crossbows and absurd when going all Spider-man with a pair of bolas. In any case, before you bring this to the table, ask the GM.
For example, this GM has no problem with an attacker who hits the same foe first with a primary-hand dagger then with an off-hand dagger to then deal the extra damage from the feat Two-weapon Rend just like it says. However, this GM would struggle to allow the same attacker getting 1½× his Strength bonus were the same series of attacks made with, for example, a pair of light crossbows—but as it's really difficult to maintain such a rate of fire in the typical campaign and an attacker would've devoted so many resources to doing this once per round, I might also find it difficult to say No.
These feats are unmentioned by the FAQ, and I couldn't locate any developer commentary on this. In fact, on Paizo's messageboards, this topic is rarely discussed at all (here's from 2010 and some crazy stuff from 2009). I suspect this is because both two-weapon fighters and thrown weapon warriors have better ways to spend their precious feats.
They apply to any weapon duo you can use with Two-Weapon Fighting style, but most projectile weapons do not get Strength bonuses on damage rolls. Then, hand crossbows would not benefit of the Strength damage boosts of those feats.
I personally think these feats are intended for melee use including its extension those one used as thrown weapons but not to projectile ones. But let's say what the literal rules say.
The Two-Weapon Fighting style and related feats are written generally open so they admit melee weapons, dual weapons, thrown weapons to be dual wielded. There is nothing said about projectile weapons but it is not explicitly excluded. Yet in the case of the hand crossbow it is specifically allowed to attack dual wielding them as per the Two-Weapon Fighting style.
The projectile weapons description on Equipment is clear though on which ones can benefit from Strength bonuses:
Projectile Weapons: Blowguns, light crossbows, slings, heavy crossbows, shortbows, composite shortbows, longbows, composite longbows, halfling sling staves, hand crossbows, and repeating crossbows are projectile weapons. Most projectile weapons require two hands to use (see specific weapon descriptions). A character gets no Strength bonus on damage rolls with a projectile weapon unless it's a specially built composite shortbow or longbow, or a sling. If the character has a penalty for low Strength, apply it to damage rolls when he uses a bow or a sling.
Thus, it seems clear the Double Slice Strength bonus will apply to thrown weapons but not to a hand crossbow.
As about Two-Weapon-Rend, the extra damage seems more open to interpretation. In particular the 1d10 does not come from a Strength damage and seems eligible to be applied with hand crossbows.
Side note: You still need both hands to load a hand crossbow, which means this is not a very effective strategy unless you have at least 3 hands.