OK lets just stop with the 2 modem idea, it won't work. So lets concentrate on what will. A better wifi extender.
Those wireless extenders plug directly into the power, unless you have power outlets near the ceiling those will never work. You need an extender that you can put up on a shelf, like this or this, although I would get one with external antennas. I am not recommending either of those over others I was just giving you examples of the type I would recommend.
I would put the DSL device up on a shelf. Then take a laptop, get it connected to your DSL device, and slowly walk away and into rooms until you lose the signal, then you'll know where it dies. Go back to where you are as far as you can get away from the DSL device but still have really good signal, it doesn't have to be full signal but it shouldn't be less than like 1 bar down and full signal would be better not going to lie. This is where I would put the extender. Up on a shelf. And you don't want to daisy chain extenders.