I have subscribers with - in some cases - two email addresses: a primary and a secondary. Is it possible to send to both email addresses in those cases where a secondary one exists?
Or do I have to enter two subscribers in this case? Having the same name details but differing emails?
Why exactly would you want to have two emails sent to one person? I'm guessing it's so that you make as sure as possible that thye get your mail, but if they have two addresses they're likely checking both of them, and if you send it to both it'll just get irritating for them as when they check the second email address they've then got an email there that they've already read.
Stick to just one.
Here's a thought: mail to the first address, but insert the second one in the email (personalisation tag) with something like: "We mailed you at [address1] but we also know you by this [address]. Prefer receiving our emails on the second address or even a totally different address? Just let us know through this form" (link to Preference Center). You might want to split or segment subscribers with two addresses so you can adapt a mailing just for them with this added feature.