Yes, mikrotik doesn't see different id tunnel connections. So, if you have one vpdn connection from one LAC, and receive another from the same LAC (with different tunnel id or with the same tunnel id), mikrotik disconects previous connection. So, this is main trouble if you shut down l2tp tunnel authentication on LAC.
If you have Cisco router, you can have as many as you want connections from the same LAC. That connections also can have the same tunnel id and works without a problem.
It will be excellent if Mikrotik can use this tehnology in next routerOS releases:
1. To make l2tp tunnel authentication possible.
2. To make possible multiple (without limit) l2tp connections from the same LAC
3. Include multilink possible with l2tp connections from LAC (so you can use ISDN 2 channels without a problem)