Short answer - no. The thing with antenna combiners is that despite the IEM transmitters being quite low power, RF transmitters are quite sensitive to reflections coming back up the feeder from an antenna. VSWR is the ratio of of oomph going out compared to it coming back up. Normally, it's just less than ideal antenna systems that don't match well and produce this reflected power. In something like a Sennheiser combiner, each input connector is fed to a buffer stage before being combined, so this active buffer presents each transmitter with a constant proper load.
A passive splitter is a combiner too, so if you stick power into it, some of it will appear at the other input terminal. Some transmitters might not care, but others could be damaged. The buffer is pretty important. It might work, but it could produce some very strange results - possibly even odd mixing and spurious outputs. Transmitters need careful treatment.
