For a start toilets only have one waste pipe.
connect what to the downstairs toilet, the upstairs one?
Problems with thiscan arise with two toilets connected to the same pipe as flush the upstairs if connected to the downstairs could syphone the downstairs one and you'll end up leaving the house because of the stink.
You will need an air/pressure cotrol vlave that prevents syphoning it opens when toilet flushed to remove pressure and no stink by the way it works.
Take a digi picture of the toilet down stairs. Go to your plumbing shop [not diy store] with the camera or pics explain clearer than you have here what you want to do and they will give you all the advice and parts you need.
Bob
March 2010