I don't understand why you would not just have one activity in Moodle. So it would be a single offline activity and in the assignment you say to the student:
"attend on Monday or Wednesday".
Or just make it an assignment and say:
"attend on Monday or Wednesday and tell me which one you attended in this assignment".
I guess I am wondering if you might need to think about your course design a little differently on a high level. Really you just have people in groups. Physical location and/or activities need to be looked at a lot differently than they do logically. There is no reason multiple groups of people can't meet in the same course at the same time. And if that is the case and you are the only instructor I think this would be very desirable!