Hi Marc,
If you're working with two disparate authentication methods, then you're likely to have to do some work to coordinate the two.
On the DNN site, you have two options. First is to write your own MembershipProvider. One option here is to override all/most of the methods, do your coordination work, and then pass off to the default AspNetMembershipProvider. Alternatively, you could simply authenticate against your second website solution and drop the Asp.net membership altogehter. The latter option is a lot more robust (it would result in one set of members) but may or may not have the necessary hooks in place to get everything done. You would also lose all of your existing AspNetMembership users (but would gain all your "second website" users).
I know there is a module out there that offers to do some of this work for you, but I do not recall offhand what it's called or where it's located. I remember reviewing it myself and concluding that it lacked the functionality that I needed to perform a similar task.
You haven't provided enough information about coordinating members on your second website back to DNN, so I can't give any guidance in that direction.
All in all, this is a moderately difficult development task. There are a lot of subtle gotchas involved, so proceed with caution!
Hope this helps!
Brandon