In our business we've got two incoming ADSL lines, one for general office and public use (let's call it Line1) and one for a card payment system (Line2). While Line2 is quite reliable, Line1 has been troublesome for years and nobody knows why and we've given up trying to find out. The current scenario is that we're using one Vigor 2820Vn for each line (the V version only being used because the routers were available quite cheap), while the one for the rather heavily used Line1 has got a connection to the other router via WAN2 and bandwidth in this case is balanced by the "Auto weight" function. This works very well but is of course only a backup in one direction.
I'm now thinking of doing the same with the 2nd router, connect its WAN2 to a LAN port of the 1st one and only activate this connection in case the ADSL of that one drops out. I'm only afraid the 1st one would then see an internet connection on its WAN2 port and start balancing its own connection which might end up in smoke or some other sort of failure.

Has anybody tried this before?