[rstanley]

"Normally it's taboo to serve to your cross court opponent in doubles, mainly because your partner doesn't have time to get out of the way and would invoke an avoidable hinder. There's a few exceptions:

When the cross court opponent is "cheating" on your Z serve to the opponent on your side. He moves over toward the center of the court instead of staying on the left to cut off your Z before it gets to his partner (if you're serving from the right to the right). In this case, if you're capable of hitting a good ace drive serve to the far left, that doesn't come off the back wall, he will not be in a position to reach it.
There's an extremely advanced serve from the right that cracks out on the left side wall/floor just behind the short line. If the ball comes out, it bounces towards the center of the court and doesn't go near your partner.
Sure you can hit Zs, lobs, and drives right to your cross court opponent to catch him off guard on the last point of a tiebreaker. And you may have given your partner and advanced signal to expect such a serve and to get out of the way, but often these points are the beneficiaries of the opponent hitting a weak return because he is afraid to hit your partner, not because the serve was so good." - from http://www.surfermall.com/rball/doubles_serves.htm