Kamara, good question. I wish I knew the answer. I had a set (3) tripytch and it didn't sale. So I separted them. Then someone was interested in a pair together and then chose another different painting.
Try including them in your 5th photo frame reference as availability as a set and put into your description listing as an option.
Also, reference the listing link inside your description for them access to the additional painting. Good luck. Would love to know the outcome. And your decision.
On another note, I listed a pair separately and one sold and I couldn't contain myself to ship it without the other. I took a loss, only financially, by including it with the other. It just told a story that I could not resist letting go without it's mate. The customer was extremely appreciative of the story as well as the gift. So, on my experience, if you believe strongly they are a set then list together.