Interesting. I don't know much about either of these sites, so i'm surious to see if this works out for you.
Consider putting short videos or even clips from your larger course on Youtube, with instructions on how to access the full course on Udemy or skillshare. People search for this "how to" stuff all the time and it would be the cheapest form of advertising you could do. Not just free, but your youtube videos can actually make money too. And if the viewers like your teaching style they would most likely be interested in an entire course on the subject, instead of looking up 50 different youtube videos.
Can't hurt anyway. Good luck.