All your attacks in a single assault phase count as coming from the same weapon, there are one or two exceptions to this rule concering some specific wargear and such, so unless Talons have some odd rule that lets them strike seperately then in the case of your guy with R-Claws and S-Talons, you would roll all your attacks as rending claws, or all your attacks as scything talons,your choice. Theres no mixing and matching. The same would go for say, a space marine sarge armed with a power sword and a power fist, he could only use one or the other weapon each phase, but in seperate phases he could use the other weapon.