Hi, Susan,
I think you should use the personal kua numbers of both partners to help them activate their personal best directions. (The fortune colors are good for them, too)
I used the kua number of my business to determine the main color of my business website and my business card. I must admit that I have no idea what else could be done with the kua number of a business - but: A business is run by people and therefore I think it is essential to establish power positions for its owners and to place activation cards and so on.
Then you have still plenty of time to determine, whether you should use the male or female number
They won't get bankrupt just because of that (Feng Shui is one third - earth luck - and there's one third human luck - behavior and attitude). And it cannot be wrong to trust your insticts, too (I chose the right colors for my business before I knew anything about Feng Shui or kua numbers)
And you don't have to be perfect. I think Feng Shui is a development, just like different clothes and colors in different stages of your life - as you grow you change your style... So maybe you should offer them several visits, just to adapt things to needs...
Enjoy!
Astrid