For sure, the active splitter is the best solution. The microphone only sees one load, which is the splitter's input impedance. Each of the outputs is buffered from the rest. The only rub is that the splitter's sonics are imposed on everyone that takes one of the split outputs. In this case, the splitter supplies phantom power to the microphone and couldn't care less what the mixers are doing.
Transformers are the next-best solution.
From a microphone loading perspective, this is no better than simply paralleling the mixer inputs. If you run into a nasty grounding situation, the transformer coupled splitter will save your bacon. The FOH mixer supplies phantom and everyone is happy. In a really nasty scene, both mixers might get transformer splits, and the splitter itself has phantom feed resistors and a 48V DC source. In many instances, simply connecting the mixers in parallel works just fine.