Learn English – “Trainer” is to “trainee” as “mentor” is to what

Quite a late answer, but Wikipedia calls such pairs "Converses" or "Relational Antonyms"

In linguistics, converses or relational antonyms are pairs of words that refer to a relationship from opposite points of view, such as parent/child or borrow/lend. The relationship between such words is called a converse relation. Converses can be understood as a pair of words where one word implies a relationship between two objects, while the other implies the existence of the same relationship when the objects are reversed. [3] Converses are sometimes referred to as complementary antonyms because an "either/or" relationship is present between them. One exists only because the other exists.

Jack of all trades is the term that immediately comes to mind. Very common: check. Doesn't connote that the person in question is particularly good at any particular trade: check. In your case, "all" just happens to amount to two, which is not a problem and would be perfectly clear from context.

Edit in response to your comment: Jacks of all trades is fine. And of course the more constraints you keep considering, the less likely it is you will get a result that's less weird, or indeed that you get a result at all.

That being said, since you are aiming for a single-word agent noun, all you need to think of is a verb to derive it from. You have viewer for people who view, listener for people who listen, and so on. Now, since you specifically didn't mention what the other type of activity was, we can't think of a verb that would mean "view + the other activity", but have to go with a very generic verb for "doing more than one thing at once". For which the most common word in my book is multitasking.

So your single-word agent noun would be multitasker.