You don't need to upload it twice. Upload it to one person, then you can tag (not sure the term) the spouse and it will show on both profiles.
Once you've uploaded the certificate, look on the right-hand side of the photo's page. Under the "In this image:" heading, you will see one spouse listed. Click the [add/edit] link under their name, then look for the "People & Things in the Image" section. Add the other spouse's wiki ID in the box labeled "Add person or free-space profile:" or select their name from the Watchlist box. Lastly, click the button that says "SAVE CHANGES TO IMAGE DETAILS". Ta da!
Here's an example for you: https://www.wikitree.com/photo/jpg/Faws-1 I uploaded the photo to Agnes, then added Floyd and Howard. As you can see, the photo is visible on all 3 profiles.
Let me know if you still have questions.