It is definitely possible to select either a company or a contact, but I think the original question is if it possible to select a company AND it's corresponding contact, for example if having a large company in which you sell to different departments and people.

I clarified my answer. Yes this is possible.

This should be fairly simple to deal with under the circumstances Andres describes I think. Under the customer drop down list under new opportunity, you can create the customer as company or select the company (if you have already created it under customers) you transact business with, meanwhile, when creating the customer, you have the option to create contacts within the Company, and they all appear as customers except when you filter. You can therefore choose the contact you wish to correspond with easily I think. Hope this helps.