SjShane recently posted on the NaughtyDog forums that you can actually get a 3rd pre-determined “buddy” booster by matching character themed weapons to their respective skins.
In order to do this, you’ll need to equip both named weapons on a loadout, as well as use the correct skin. Each character set also gives a different booster:
Drake + Drake’s AK + Drake’s Para 9 = Explosive Buddy
Sully + Sully’s Dragon + Sully’s Arm Micro = For My Buddy
Elena + Elena’s G-Mal + Elena’s Raffica = Thanks Buddy
Chloe + Chloe’s M9 + Chloe’s Tau = For My Buddy
Marlowe + Marlowe’s M9 + Marlowe’s Para 9 = Sugar Buddy
Talbot + Talbot’s Fal-SS + Talbot’s Arm Micro = Stealth Buddy
Rameses + Rameses’ Kal-7 + Rameses’ Raffica = Booty Buddy
Note that only the first and original skin of each character will work, variants such as Suited/Desert Drake, Young Sully, Desert Talbot will not activate the bonus booster. The Jade Weapon set does not give any bonuses.
This combination also requires a named Long Gun and Side Arm, so one offs like Cutter’s KAL-7, Flynn’s M9 etc will also not give any bonuses.
Credits go to SjShane for posting this. And Cloudtail4ever on GameFAQs for the image.
