FCC certification for multiple transmitters
You will need to pass FCC 47, section 15 to show that you conform to the EMC standard for the USA. The tests are required for your product, as it is an intentional emitter, as well in an unintentional emitter.

If you are looking at other markets, you'll see you have other standards which you'll have to meet. Pick up your mouse, keyboard, monitor, computer, and you'll see a plethora of other logos along with FCC: CE, CASA, CAS and others. There is a different set for different countries, or groups of countries, most are defined in this document here: https://www.ieee.li/pdf/essay/guide_to_global_emc_requirements_2007.pdf

In answer to the "how do other companies deal with this" is they do lots of testing and follow any read-across that they can find. Most test houses will be happy to help with this.

Don't forget, if the IoT device is to go in a car, it will need to conform to the automotive standards (which has even more variation than home equipment) and there are also medical and military standards which may have an influence depending on your device and where it ends up.

My advice is to contact a local test house and ask them what they recommend for your target market.