How can I dis-allow both a quantity discount and promo code on an order form?

We have a simple order form for a workshop. We offer a quantity discount (20%) for 2 or more registrations. And we have some different promo codes all using the Category code since we can have two products in the order form and we want the promo/discount to apply to each.

Some of the codes we want to work in combination with the quantity discount, and some we don’t.

For instance, the 10% off promo code plus the 20% discount together for 30% off of off multiple registrations is fine. We want this business.

However, we have a 50% off promo code for friends and family that we do not want to be combined with the quantity discount. But we can’t see a way to do this.