IRIS for Industry Companies working in Partnership

In this answer I assume that by "combination product" you are not referring to a "fixed dose combination product", since this last one would have a single RPI rather than two (RPIs are assigned for development products, substances are mastered in SMS). 
In IRIS, it is not possible to add an RPI assigned to another Organization (legal entity) to the submission data, even if the other Organization belongs to the same multinational group. This is to ensure the necessary confidentiality of the RPI data that are used and displayed in the submission.

• In Orphan designations, only one product can be the subject of a procedure.
• In a Scientific Advice submission, additional RPIs can be chosen but they must belong to the same Organization; additional products for a combination treatment, but belonging to a different Organization, can be mentioned and discussed in the Scientific Document that will be uploaded with the submission.

Not sure if this is of relevance here, but during Tuesday's EMA webinar on IRIS & Scientific Advice there was also a screen shown with the statement that "... if the new RPI is not being requested for a single medicinal product ... but for multiple products, .... the [RPI] request should be sent directly to the relevant EMA service (e.g. ... scientificadvice@ema.europa.eu ...)
 

RPIs that refer to multiple products are generally used only for ITF submissions.