Witches have the ability to siphon Mana from other individuals to add power to a spell. They accomplish this through specially designed runes, which are tattooed to an individual. This rune links the person to the witch, who can then draw Mana out through the metaphysical link. This method is not without its dangers however. Drawing out too much power can harm or even kill the subject. Therefore, safety standards have required that witches limit their siphoning to 40% of the person's Mana supply or less.
Evil witches naturally wouldn't care about safety, and would gladly sacrifice a victim for power. For more scrupulous witches however, this stipulation naturally blocks witches from accessing the most powerful spells, as they normally require more power than witches usually have. Research has developed a possible workaround. By placing runes on multiple sources, a witch can draw Mana from several individuals at a time. This can allow them to bypass this rule without potentially killing someone by using more than one person at a time, drawing up to 40% from each.
I would like to limit access to the most powerful spells to dark witches. Therefore, I need for this method of siphoning power from multiple sources to not be a viable alternative to the normal method. How can I prevent "good" witches from gaming the system ?