I use a splitter and do not seem to have a problem with range. We 'see' ships at more than 20 miles. Certainly not a scientific experiment since there is no control for comparison with two antennas. The idea of having a redundant antenna is attractive though.
In South Africa doing lots of boat stuff. Departure north from Cape Town around December 15th. Arrival in the Caribbean around the beginning of February, after 5300 miles or so.