Greetings,
Without relinquishing ownership of the original sharer’s EndNote online account, colleague (A) would have to send a copy of his/her synced EndNote library to colleague (B), so that colleague (B) could synchronize the library with his/her EndNote online account. After syncing the library with EndNote online, colleague (B) would have to send new sharing invitations to all sharees that were part of this collaboration team to mimic the original library sharing list.
Please let me know if this helps.