Individual authors taxes are not a matter envato must be concerned of: once you withdraw money from your account, whatever the source (sales, referrals, author to author transfer), it’s up to you to declare that income and pay the related taxes.
May be Envato directly splitting each sale at a constant rate between 2 author could be legally possible (apparently). But author to author transfer is absolutely different service! You can’t just display that on statements page (like “Bob sent $440”) and consider that as clean proof of income.
Coming to the each sale split % method, anyone has idea about if a copyright issue arises and both the authors resides in different countries? We already have enough complexities related to license and copyright.