In an effort to have Assassin's Guild Members keep their identities a secret, but to keep guild assassins from having to kill EACHOTHER... I believe Guild Assassins should not gain benefit from targeting the corpses of other assassins with their dagger. HOWEVER:
"Bounties" placed on members of the guild? Incur a debt the assassin must work off (or pay off) that comes out of their pay when they do assassination contracts before they can be paid for their work, requiring them to not get caught and not be identified while committing assassinations - with normal bounty decay so that if it lasts untouched over a month, they're free to work again. This also means that assassins can be paid by fellow assassins for helping with their contracts - who's identities are not as widely known - if they need coin badly.
This helps prevent assassins from intentionally "losing" in "fights" with other assassins, to get each other coin and then casually working with each other again after that, and incentivizes more natural behavior among assassins trying to keep their identities as guild members a secret without expecting a breaking up of their relationships to each other in a limited-sized pool of co-workers.