You are not obligated to pay JW LEGACY any mechanical royalties, but you are free to do so if you desire.
However, as to be found in the license agreement, the shares of ownership (writer's share, publisher's share etc.) is split 50/50 between JW LEGACY and the artist.
This is the standard when it comes to collaboration online. If you have any questions relating this subject, feel free to ask me and I can provide you with the necessary information.