I have steam and this is a legal copy. I currently have both of these add-ons downloaded but when i try to install one add-on into the game(I only check one of the add-ons when i play) the add-on never works. I have tried single player and my own private lobby survival but nothing works can someone help me?

Try taking one of them out.

I only have one checked off in my addons and they still won't work.

Are you going to a server that allows skin mods? Try making a local server and test to see.

Just because you have it unchecked in your addons, does not mean they won't conflict with each other. Try what Gunzers6 said and remove one from your addons folder and test it again.