The standard diversity antenna that is included with some composite output 12v TV tuners (or FM) tend to use twin jack input, so yes I would have thought the original aim was to source the signal from either.
But, to test any theory further I have tried these antennas in series and paralell. The most succesful was paralell. I have just used a mini-BNC or Coax combiner. i.e. reverse a splitter. Or just bodged the two together.
Back in the day of my "normal" multimedia install the modification above worked very well.
I installed a Freeview Car-TV tuner the other day that came with a crappy antenna that is even worse than anything I have tried.
I think I may just attach a high gain, silver, shiny, large.........chimney antenna to my roof!