I am studying a simple educational environment where there are two types of activities (call them A and B). There are three types of students: those who only engage in A, those who only engage B, and those who engage in both. It's easy enough to name the first two categories (I name them after the activity types, e.g. "viewers" for those who view lectures), but I'm struggling to come up with a good way to refer to the third type of student. What is a relatively common word that roughly means "student who engages in both types of activities", but doesn't connote that he/she is particularly good at them?
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.
There probably is no such term. You could say something like multitaskers or dual participants, but perhaps the best approach is @J.R's suggestion to join the two actions using a hyphen.
Of course, this is assuming, they're not going to start working on activities C and D soon :-)
To begin with I would suggest it is not wise to use the terms A and B (which you may not in any case be doing) as that suggests that A is in some way superior to B, which notion you may not wish to convey.
Irrespective of that I would call them:
Viewers/ Practitioners / Combined Operations.
Degrees which combine two subjects, such as 'Modern History with Politics', Economics and Social Science' etc. are usually called 'Combined Degrees'.
In American football, a two-way player plays both offense and defense.
(If you don't know American football, teams generally have entirely different platoons for offense and defense.)
So you could call these students two-way students.