The 5 pin GPIO connector SDA and SCL pins are the same I2C connections as on the FFC connector to the Raspberry Pi, there is only 1 I2C port. This port is used for touch events and backlight control by the Raspberry Pi GPU driver
Controlling the display backlight brightness via the I2C pins
Some users report this is possible but others that it can stop the touch sensing working and also cause problems if there are other I2C devices on the bus. This isn’t surprising as the GPU driver is accessing this I2C port as I2C master. By trying to also access is from your own separate master I2C port you are creating a multi master I2C bus and if the GPU driver or your I2C port driver hasn’t been implemented to support multi master arbitration then collisions will occur and break comms.