I don't do any arts subjects, but for my subjects the workload has definitely increased, no question.
I'd say it depends on what you get at AS. If you manage to nail all 4 subjects this year (and have some UMS in hand), you might as well try all 4 at A2, and see how you get on. Anyway you can always drop one anytime.
It's really how much time you're willing to give up for the extra subject, and I'm afraid you'll have to do some work during free periods. Be prepared to become very cynical, and feel like dropping out of school to work fulltime in Tescos, at some point during the year.