Rory,
Welcome to the RDM forums!
Yes, you can definitely have multiple secondary slots pointing to the same primary. It can be quite common on moving lights with gobo wheels and color systems too. There is a good webinar we did on this subject a couple years ago you can find here that I think will hopefully provide a lot of insight and some examples of this.
https://www.youtube.com/watch?v=-5BmsJ7CeFE