So - IF I have two (or more) I2C devices working with different levels, then one of them must be used with level shifter. It is clear.
The question is - "which strategy is better"
A.\ use 3.3V device (eg SI5351) without shifter (like Raduino) and other device (5V) shift to 3.3 and then connect to raduino (exactly "to I2C pins of si5351")
B.\ cut traces to si5351 at Raduino, insert level shifter, shift SI to 5V and then use OTHER device(5V) without shifting. (connecting to 5v I2C pins of arduino)