Maybe it could be done across different schools, different classes or different years. For example, in year 1 teach subject focusing on quality and subject 2 focusing on quantity. Then in year 2 reverse the roles. But then you also need to be careful with the order of the subjects.
Just splitting the students into two groups would be better though, aside from the complaints. This is a problem with A/B testing in general: people want to be treated fairly. Are there good ways to reduce (the risk of) such complaints?
To me, the simplest solution that comes to mind is to grade on a curve at the end of the course, based on the quality of the work (or some other subjective measure - I'm not familiar with university art courses, but I assume there's some kind of widely-accepted grading methodology). That is to say - tell the students before the course they will be graded on quality or quantity depending on the group, but grade each student on a curve relative to their own section for their permanent grade once the course has finished. This would obviously still require lying to the students (at the very least by omission, arguably), however.
Followup to: Don't Fear Failure
In the same theme as the last article, I think that failure is actually pretty important in learning. Rationality needs data, and trying is a good source of it.
When you're trying to do something new, you probably won't be able to do it right the first time. Even if you obsess over it. Jeff Atwood is a programmer who says Quantity Always Trumps Quality
The people who tried more did better, even though they failed more too. Of course you shouldn't try to fail, but you shouldn't let the fear of it stop you from tyring.
I wouldn't go as far as to say that quantity always trumps quality, but where the cost of failure is low lots of failures that you pay attention to is a pretty good way of learning. You should hold off on proposing solutions, but you also need to get around to actually trying the proposed solution.
I'm normed such that I'll spend more time talking about if something will work than trying it out to see if it works. The problem is that if you don't know about something already, your thoughts about what will work aren't going to be particularly accurate. Trying something will very conclusively demonstrate if something works or not.
Note:
I originally had this as part of Don't Fear Failure, but that post got too long.