And I just thought of this -- benzos work short-term and are addictive, while anti-depressants work all the time. So benzos won't cover you all the time anyway but an antidepressant, if it's working, will, so the antidepressant might cover for the absence of the benzo. When I was on Paxil, it was very sedating, and my klonopin was cut from 1mg to one half twice a day. I never noticed a thing, probably because the Paxil was covering it. Doesn't mean this will happen with you, but I thought I'd mention it.
My advice, and it's only advice, is to taper one completely before going off the other. Since you've been on the Zoloft only a few months, it should theoretically be easier if you were on the valium a long time, but not necessarily. This is a very individual thing, as both can have long-term or permanent effects on the brain or no effects at all -- it runs the gamut. But doing two at once would seem to just be asking for trouble.