Okay, well how are you directed to take them both? Is one as needed, both as needed, and if so, how do you know when to take which? Do you take one of them regularly? It's pretty uncommon to be prescribed two short-acting benzos concurrently.
The equivalency between the two meds isn't an exact science, but per equivalency charts, approximately 0.5mg of Xanax equals 1mg of Ativan, so the Xanax is roughly twice as potent, mg to mg. Again, not an exact science.