The limitation of a single active installation process is 'hidden' in the API reference

[athoelke]

In the description of psa_fwu_install (function), it states that only one set of components can be in the process of being installed at any one time. And only when the current set of components have been installed or rejected (no longer in STAGED, TRIAL, or REJECTED state), can a new set be installed.

This behavior is essential for correct and secure operation of the update process, and should also be clearly described somewhere in §4 Programming model chapter.