I found it interesting that the word zealous automatically made me think of the word jealous. One is a very positive action, while the other is very negative emotion.
You could have someone that was zealously jealous. Taking extreme actions to express their emotionally jealousy.
But I am not sure you could have someone that was jealously zealous. This seems that someone is still showing zealous actions but expressing in a jealous way.
I would suggest avoiding both types of person as being somewhat unbalanced.