Not a native speaker either, but I spent a fair amount of time in the US and Canada; and I don't think I've ever heard that phrase. Usually it's "buy one, get one free" or some variation of that.
The phrase you mentioned would mean that you receive 2 units and pay for 1.5.
"Buy one, get the second half off."
Is that meant as the second unit you buy, you get half off? Don't know if this is a common way to phrase a discount like that in the US..