When listing a diptych, triptych or any panelled work, you will need to list all the individual panels together.
This means that the measurements shown in the listing need to be the total of all the panels combined. For example:
a diptych that is is two canvases that are 40cm W x 40cm H each and are to be displayed next to each other horizontally will need to be listed as 80cm W x 40cm H
a triptych that is three canvases that are 25cm W x 25cm H each and are to be displayed vertically will need to be listed as 25cm W x 75cm H
You will also need to make sure that your primary photo shows all the works together as they intend to be displayed, and that a photo of each individual panel is also included in the listing.
Use the artwork description to say that the work has multiple pieces and list the individual panel measurements and any other special requirements.
See the below screenshots as a good example.
