Need a bit more information on exactly what you are trying to do. The main issue is that each user is specifically linked to an individual tenant so changing the tenant means you also have to use a different user that belongs to that tenant.
You can issue a TenantSwitch however as the user exists against the old tenant you will then effectively be unauthorised. You can use the Login function from within System that accepts just a user ID (Not the User_login or LoginPassword actions) however that will have the wrong tenant ID so won't work for you.
Depending on what you are actually trying to achieve you may need to create multiple users in each tenant by the same name, change tenants, search for the user, get it's ID, then use the Login function to login as that user without the password.
But having to do this implies that you aren't using the multi tenant functions they way they are intended. The idea of having multi tenants is to completely separate different users or "clients" so having one user jumping between tenants doesn't work very well, particularly as the users roles and security have to be redefined in each tenant.