I2C can be routed as a diff pair, but with I2C you need to take into account "CrossTalk" between the clock and data line. You would want them separated so that crosstalk between the two lines is minimal. The actual buss speed for i2c is very slow. Something like an 8mil trace with a spacing of 15 mil between SDA & SCL would be good. From the perspective of crosstalk you would be looking at an attenuation level of about -40dB for a risetime of 300ns in either I2C fast or Highspeed mode. One note, when using i2c you would want those SDA and SCL lines running over a ground plane or reference plane. Don't forget your pullup resistors :) All the best.
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