If you are going to use splitter transformers make sure there is no phantom power being used on any of the mics. If there is make sure you have splitters that pass phantom. ISO transformers with ground lift are also helpful when interfacing two systems together. Also if you touch the FOH guys gear the old adage " you break it you buy it" goes into effect. Any noise or interuptions get laid off on you. Set up early, test early and then a full FAX right before showtime.