I have not had any experience in defining the Primary Mode of Action (PMOA) for a combination device. The best way to define the device is by analyzing the two components involved in the product. Based on this, it will narrow down the type of designation the device will be given. In addition, the FDA has Request for Designation that can be filled as someone has previously mentioned. With this form, it allows for a request to identify the designation of the product. This would be the most effective way to identify the PMOA. This would then ensure that the product follows the regulations presented by the FDA.

https://www.fda.gov/RegulatoryInformation/Guidances/ucm126053.htm#IIB