I am struggling with a similar issue.  I want to use two of the Fermion MAX30102 sensors with a multiplexer and can use the https://github.com/DFRobot/DFRobot_BloodOxygen_S library and I have success.  However, I want to control the LEDs of each one and there is nothing in this library which allows this.  If I use any other library, I get “Sensor not found” message.  Is there a solution?  Can I modify the library?  (Not sure I have that ability.)