If I work for two companies and get trained as an RA agent by one of the companies, am I allowed to act as an RA agent for both or only for one of the companies?

You may act for the company with a valid RA agency contract with Swisscom and whose Master RA agent has appointed you as an RA agent. The prerequisite in each case is that they have a current contract with this company, e.g., an employment contract, temporary or fixed, a project assignment, or similar. If a person works for two companies, then both companies must have an RA agency contract, and that person must have an RA agent role in both RA agencies so that they can also identify people for both RA agencies.

Just so you know, as an RA agent, you can identify people as you wish, including people from outside companies or individuals. Having non-company RA agents working for an RA agency is not allowed.