Using higher I2C voltage then MCU supply (but still in limits)
Your proposal may work as long as you connect the I2C pull-up resistors to 2.8V (the lower voltage) for both I2C1 and I2C2. This way, both I2C busses would be working at 2.8V. 

You have to check how much margin you have for I2C2's VOH when the pull-up resistors drive the lines to 2.8V while the I2C peripheral expects 3.3V.

My recommendation is to simply use level shifters with N-FETs. They are extremely simple and work really well.

The following references show how to implement them:

I2C level shifting NXP application note

SparkFun I2C level shifter schematics