The poster already has 2 ADSL (hes kinda made his choice). What i suggested would be the best way to make use of them both (OR RATHER best way to give options on how you can use them both).
YOUR suggestion right back at the beginning of the thread of
"a 2830N and an additional 120 modem connected to WAN2." would have course also work and be a solution. Thats pretty much what visibleman posted as a diagram if the poster needs that to understand also
I agree if the phone pole miraculously blew/got knocked over though or a cab got flooded he would be screwed, not much i or we can do about that now he has already choose 2 ADSL lines. I just gave the best solution to what he has and what he asked for. For true resiliency then yes something like a 3G backup solution would be a good idea, but he doesnt (As far as we know) have that. Your idea and mine would work just fine though with what hes got

both entirely viable
The same cable? what on earth are you talking about he has 2 phone lines, Do you mean the same bunch of pairs running to the exchange? Otherwise if he lost one phone line its unlikely to affect the other. Load balancing means he could use one or the other or both. It gives the most options for what he has.
For him to lose both it would likely affect more than him it would likely affect a whole segment of his street if it were a pole issue (probably a dozen homes all out if the pole got knocked out) or if it were a cabinet issue that will affect over 200 people. (standard cabinet has 288 lines).
It will not protect him from DSLAM "FAILURE" either, if the DSLAM "FAILS" that will take more than just his lines out. In fact him being ADSL and thus the card is at the exchange not the cabinet it will take out hundreds if that TOTALLY FAILS......... As opposed to port failure which is probably what you meant which can affect a single line. (again leaving his other line working).
His aim from his original post and 2 follow ups is quite clear. Maybe you should come up with some viable alternatives you have had no suggestions
of your own. Then again i doubt ideas are your strong point.