Hello guys, here is the situation:

We have a configuration with AD several forests (let's say Forest ID n°1 and Forest ID n°2).
We have a set of users accessing the Internet on a server connected to Forest ID n°1 and a set of users accessing the Internet on a server connected to Forest ID n°2.
We want to know if a forest ID n°1 user can connect to the second app even if he is not in the forest N°2.

Yes it's possible to authenticate users belonging to several AD forests in using Visual Guard.

Federate several Active Directory repositories belonging to distinct networks or companies. Administrators declare Windows accounts or Windows groups from these Active Directories in a central Visual Guard Repository. Then, the corresponding users can access the applications secured by the system. As a result, you get one central security system, although users are spread over several independent Windows domains.

Visual Guard 5.x