Hello Emily,
That's a tough one to address in the product as it stands.
Here's some stuff to think about:
If a given company has 5 leads in your database, would you want to mail the same 2 every time and exclude the same 3? If so, you could try using a custom field to designate your preferred company contacts, and add this to your campaign smart lists.
Will you be mailing the same people every time, or do you do a lot of changing lists and smart lists? If you're mailing the same people every time, Adam's suggestion above to simply create a static list of your preferred contacts is probably a decent way to go. This is a lot of work to set up if you expect to need many different lists, but it's not too bad if you're doing it once.