If you upload a print-ready file you should submit one single PDF document containing all pages - both front and back for a 2-sided card and both outer and inner pages for a folded card.
For standard 2-sided cards, the first page of the design indicates the front and the second page is the back.
Business cards can be designed in both horizontal and vertical orientations.
For folded cards, on the vertical orientation page, 1 in the editor represents the front and the backside.
Page 2 shows inside pages of the folded card:
The same principle applies to pages 1 and 2 when designing folded cards with horizontal orientation: