Problem with VPN client

hi
i have a problem with l2tp vpn on my mikrotik server
although I put the shared user on 2 in the userman profile, every users is only able to make one connection on per public ip.
Please let me know what should I do in order to make 2 connection from each public IP?
For instance, the user wants to have a connection in the cellphone as well as laptop at the same time but there is only one public IP on modem