You're more than allowed to post your submission to both platforms, so long as you follow the criteria for posting both (on ao3, post to the collection, and on twt, @ the event acc and add the hashtag #ddsapphicweek)! On a similar note, participants are welcome to crosspost submissions to any other platforms (ie tumblr), so long as they also post the submissions to twitter or AO3 so I can boost the submission!