I made a post about this in Facebook with screenshots. Check out the tips page for this and more tips for the game. To answer your question, You can only apply one debuff at a time. If you apply enemy health debuff then apply enemy defense debuff then it will replace the enemy health debuff. Once applied you will get the benefit both attacking or defending. While a debuff cannot be used with another debuff you can still combine with a boost. For example, you could combine hit point debuff and attack boost. There is a screenshot in facebook that is color coded with the items you can combine. Check it out and drop a like.
Hammer Nails