Hi Roland,
I highly doubt this will work in your scenario based on my recent experience with WSS.
If I understand you correctly, in your scenario, you have two identical forest and having a two-way domain trust relationship. Recently, I have come across a very similar situation where Auth Connector is installed on domain ABC.local and it has a two-way trust with XYZ.local. Unfortunately, Auth connector didn't recognize any of the user that are authenticating from XYZ.local, WSS reported all the users that coming from XYZ.local as "Unauthenticated User".
While installing Auth Connector, we have two options, the first option indicates that it is going to query all domain controllers. In our case Auth managed to syncronized users from both domains. But it won't recognize user credentials who is coming from XYZ.local. We have not tested the ACLogon option (the second).

See Symantec documentation here:
https://portal.threatpulse.com/docs/sol/AccessMethods/auth/authconn_config_ta.htm
To summarize: I hightly doubt users that are authenticating from the other domain will be recognized. However, I would appreciate if a Symantec Engineer can confirm the outcome if they have had tested such scenario. If you test this and if this works please let us know.
How about installing Auth Connector in both domain?
Regards,