Currently, there appears to be no way to sign up to beta test for multiple platforms, nor is there a way to change the platform once one is selected.
Is this intentional design? If so, will there be a way to at least change the platform at some point in the future? Initially I tried to sign up for both PS4 and PC and the website seemed to allow for it. However I only got email confirmation for the PS4 and visiting the page again it notifies that I've already signed up and offers no option to change platform.
In my experience with previous Ubi betas, you have to select a single platform for a single Ubisoft account. If you would prefer to change to a different platform, you may be able to do so by creating a ticket at https://support.ubi.com .