In many cases user name and password are used for billing if you have caps on how much traffic you can do like pay per use, or if you have special services associated like unlimited traffic.
Sometimes they might use the same information for network authentication and in that case it might be associated with your cable router / ads DSLAM.
But if something nasty is done using the networks both phone numbers will be in scope for investigation as they will be correlated what phone number was used for the account they will notice 2 coper numbers associated to the user name.
Also you will be probably violating your terms of usage with the ISP.
Might be considered crime if you mess with the billing system Or fraud if you enable a paying customer to pay less.
I strongly advise you not to go that path and just satisfy the curiosity of your question with the answers here provided do not try it.
If you share the same DSLAM the probability of being able to do what you wish is high and it depends on the service provider settings.
You are always trackable in any situation but you will cause the ISP security to be more angry as they will have extra work... :P
Normally it is not possible to change credentials even if you share the same ISP but you hare connected to different DSLAMs...