two l2tp connections from same ip address

[sindy]

I have two routerboard behind the same ISP dsl router
Each of them is configured to build an l2tp connection to THE SAME server (another routerboard device) so both come from with the same public ip address
Each l2tp is configured with its own name, secret and remote address
When the first l2tp session is up , the second one connects and then disconnects after few seconds (same caller id/ip address shown in ppp active connections)

Is this an expected behavior ?
How can I sove this problem ?

Check my recent topic dealing with this for explanation why that happens and for a generic solution.
However, in your particular case it seems to me that use of manual IPsec configuration and, if necessary, a tunneling protocol of your choice (ipip, gre) instead of L2TP/IPsec would be a better solution.