Hello Jerome,

follow the procedure described here:

http://www.cisco.com/en/US/tech/tk175/tk15/technologies_configuration_example09186a008071a7c2.shtml

>> In each case, my modem is synchronize to the DSLAM

this means both lines are up/p at DSL level, but the two lines can have different encapsulations for example and you need two modems one for line1 and one for line2.

see also

http://www.faqs.org/rfcs/rfc2684.html

>> My ISP think is a collect problem (the first is probably collected in IP the second in ATM). In this case what is the solution to doing that (I have no ATM interface on my router..).

this leads to possible encapsulation problems

in short if  in the following debug command output

debug ppp negotiation

debug ppp authentication

if you never see a line that contains a capitol i I it means the router is sending messages but no one answers

Hope to help

Giuseppe