Of course! we don't ask any exclusivity, the right of publication remains with the author. If you want to use another publishing platform, just be sure to avoid distributing the same book twice to the same store.
The only requirement is to respect the minimum price policy. You mustn't post the book on other sites at a lower price than the one used on StreetLib (otherwise we must automatically lower the price in our distribution system, and consequently in stores connected to us).