What camera did you use with this lens?

Part of it is sequential, part of it is alpha-numeric: https://fujifilm-x.com/en-us/exposure-center/understanding-file-numbers-and-folders/ I do not have a X-T2, so I cannot answer your question, but as I recall you have various backup options you set for the two slots that affect what happens. Someone over in the X-T2 forum should be able to help you.

Hello. I received XF 18mm f1.4 today. It feels like at least one of the elements inside is loose. When I flip the lens 90 degrees I can hear it drop back. But once I turn the camera on I don’t see this behavior. Is this normal? If yes, I like to learn what’s causing it. Thank you. 

So if I create this folder, will it be mirrored on the second SD back up?

It does have this option, bit of a faff though, having to enter a five digit name. It would be simpler if it just did it with sequential naming.