Assuming it would work the way you describe, Nightblades would have a great advantage on the task.Dark Brotherhood:
You increase your skill rank by completing Dark Brotherhood contracts, which get you to kill a random NPC. Your skill progress is based on your detection, whether you were seen by guards, killing the right target, etc. You can't kill anyone else between killing your target and returning to the contract giver. Getting caught by the guards fails your contract and, possibly, lowers your rank.