Hi Juan,
This link will show you how to configure L2L tunnels and vpn client on the same firewall.
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a0080094cea.shtml
This sample config uses a TACACS server to authenticate users. If you want to configure local authentication you need this:
1.- PIX code 6.3.x
2.- Configure username and password entries for each client:
username cisco password 123445566
2.- Configure this line:
crypto map test client authentication LOCAL
Finally, please bear in mind that the vpn clients won't be able to talk to the network on the other side of the L2L tunnel since the firewall cannot redirect traffic as a router does.
Hope this helps.
Francisco