I would ride two separate days so that the breaks weren't too long, I'd also look into a potential part lease where the other person rides once or twice a week (depending on his athleticism as a 17yo) so that he's got more consistent work.
Two separate days so that can rest and get back into it strong
I'd ride two days in a row. That way you can do lighter work on the first, as he will be coming back from a long period of no riding, and get him ready for a more intense ride on the next day.
If he was my horse, I would ride 2 different days. But if possible I would try to turn him out the days I didn't ride, so he can stretch his legs!