You can't parallel zeners to get a higher power rating unless you match their voltage drops under load, and then it's a bit dicey. If you want a higher power rating, divide the required voltage by the number of zeners you need to get the right power rating. For example, two 30V, 5W zeners in series will be roughly the same as one 60V, 10W zener. But watch the power dissipated - ignore the derating curves and thermal resistances at your own peril...
Tranzorbs, by the way, are ineffective if paralleled and not perfectly matched. Well, they actually have the same power rating as a single one. The one that turns on first will conduct, the others won't. If the spike is really, really, big, they will all fail in a very convincing manner, as each one fails, shorts, vaporizes, and opens in sequence.