This topic has been discussed before but I wanted to mention it again for new folks! If you decide to collaborate with someone at 50/90 it is a good idea to decide in advance what that collaboration means!
1) is this just a collaboration for fun for the songwriting challenge and after the challenge both/all parties retain all rights to their individual contributions?
2) is this a more formal collaboration where both/all parties intend to create something together meaning after the challenge the entire finished product is co-owned by all participants?
This is not typically an issue of huge import but kinda nice to at least consider what you prefer prior to doing it!