Because Submittable has changed their interface, I'd like to step you through the file-adding process WITH PICTURES!! (In another life, I wrote software documentation, so thank you all for the trip down memory lane.)
1. You have opened your submission, added your first file, and hit "Submit." This means that the Zoetic staff can see it and keep track of your progress. But you can't add files!
2. Zoetic is notified that your submission has arrived. There is a solemn ceremony of thanksgiving. Then, we open it for editing. When you go into "My Submissions" and select your submission for WLYA, it should now look like this.
3. You are now ready to add your next file. Hit the orange "Choose file" button and add your next file. HERE'S THE TRICKY BIT: Submittable has now changed that little gray button in the bottom right to "Done." YOU ARE NOT DONE. After you have uploaded your next file, just exit your submission. Close the tab, choose another submission, whatever. Just don't hit "Done."
4. Here is what the Zoetic crew will see after you've uploaded your next file WITHOUT HITTING DONE. There are now two files, and we're off to the races!
I hope this has helped! If not, please remember that we're always (and by "always," I mean roughly from 8am to 8pm Pacific time) here to answer your questions!