FCC Co-Location of antennas
Modules are typically certified with the assumption that they are the only intentionally radiating device. Some module datasheets will make this known with statements like:


  This transmitter
  must not be co-located or operating in conjunction with any other antenna or transmitter.


Note, it says with any other antenna or transmitter. So, even if you figured out a way to share the same antenna, there would still be an issue.

I'm not an expert on the regulation side but I'm assuming it's not even possible to certify a module in a multi-transmitter configuration without knowing the entire configuration ahead of time.

It's important to talk to the module manufacturer, test houses and maybe even a consultant/lawyer that is an expert in this particular area so that you're not violating any FCC rules/regulations.

UPDATE: I've personally been working with TI on a similar situation and they will provide (through application) a letter that will allow us to use their data on our own FCC ID. There is still additional testing required due to multiple transmitters but the effort/cost is going to be much lower than trying to certify from scratch. Another option is request the manufacturer to file for a Class II Permissive Change on their FCC ID to allow your multi-transmitter configuration. However, I've yet to come across a manufacturer that is willing to do this.