"I am a man and she is a woman."
Translation:Ja jestem mężczyzną, a ona jest kobietą.
"a" is the version of "i" that shows contrast. Sometimes both are possible (changing the meaning), but not here. But I cannot think of such a context right now. Anyway, if X is a man and Y is a woman, that's definitely contrast.
Imagine "Ja jestem mężczyzną i on (też) jest mężczyzną". There is no contrast, two men here. The sentence is rather a strange thing to say, but it's correct.
"Ja piję wodę i jem kanapki" - just two things that I do, no contrast.
"Ja piję wodę a ty jesz kanapki" - two people doing different things, contrast.
I'm not sure whether i co? would work here. To co? and no to co? are possible, but those collocations can also mean something like "so how about..." introducing a proposition. And frankly, that's how I personally would use them.
I'd recommend the phrases i co z tego? or no i co z tego?. They are more expressive and completely unambiguous.
On a side note, if you should ever come across the similar expression "a co?" that would mean "why do you ask?".