Right, though I don't think uniform is the necessary condition here. We only need a positive chance of picking an R so that f(A) and f(B) are on different sides of R, which isn't as much of a problem if we're picking R from (-1, 1).
One way to resolve this problem is to use a continuous, increasing map f from the reals to say, (-1, 1). Something like arctan(x)/pi. You can then pick a number randomly from (-1, 1), then compare that with f(A), rather than a random real with A.