My doc started me on 20mg paxil (paroxetine). I was on 100mg zoloft. He said to take 50mg Zoloft as well for 2 weeks then stop, to prevent withdrawl. Is this true?

Yes. It is best to do a cross taper/ titration when changing medications. Go slowly to avoid withdrawal .
Sounds reasonable. While both Paxil (paroxetine) and zoloft are serotonin reuptake inhibitors and one would not expect to see much in the way of serotonin discontinuation symptoms in moving directly from one to the other, it can further decrease the likelihood of any problems by cross tapering the medications as your doctors suggests.
Cross-titration. Your doctor is being gradual in changing from one ssri to another, through a transition process. Theoretically, one ssri should "cover" for another in such a switch, but "discontinuation" symptoms (drug company term for what others call "withdrawal") are truly uncomfortable & best avoided if possible. It's healthy to ask questions -- but is there some reason you doubt your doctor's suggestions?