Hmm, interesting example.
People routinely write, "I have a brown and a black horse."
I suppose you could say that logically there are two horses, so you should use the plural. If you said it without the article, you should certainly use the plural: "I have brown and black horses."
Normally when there's a compound noun like that we use the plural: "Ginger and Wildfire are my favorite horses." "Ginger and Wildfire is ..." would be called an error in any English class I've ever been in.
If you used numbers it might be iffier: I think most English-speaking people would write, "I have two brown and one black horses". But I think most would say, "I have one brown and one black horse", not "one brown and one black horses".
It seems that when you give an explicit number of "one" with "a", "the", or "one", suddenly two ones are still considered singular.
Anybody know a general rule for this?