Does the image file I submit to deviantART have to be the same file I submit to Prints?

No, not necessarily. But when you submit a deviation intending to make it available as a print, you can initially upload a hi-res print source image to the file deck and resize it in the deviation submission process for deviation display.

In any other case, they should most definitely be different:

Images for display on deviantART (deviations) should be saved at lower resolutions/file sizes since they will be viewed on screen only.

Images submitted to Prints should be at a higher resolution that is more suitable for printing, may be level adjusted for print, and should not contain any watermarks. This high resolution file will not be available to the public.