We do not recommend trying what you are asking. Doing so will void your warranty on not just our product but that of the Soundcraft if any damage were to occur as a result. Whatever you do is at your own risk.
The proper solution to split a microphone signal between two mixers is to use a Microphone Splitter that is transformer isolated. ART Electronics offers a Microphone Splitter. Or contact your pro music dealer for help in finding you a safe solution.