The new sensors come with reduced pixel sizes, the ISOCELL Slim 2X7 being the world's first mobile image sensor with 0.9 μm per pixel. These allow for better results in low-light environments and provide improved details with increased resolution. Galaxy S9 and Note 9 are expected to feature both sensors.
https://www.notebookcheck.net/Samsung-Galaxy-S9-to-integrate-the-12-MP-ISOCELL-Fast-2L9-and-24-MP-Slim-2X7-Camera-Sensors.258016.0.html