I2C max pull-up resistors and current sink
Yes, you could sink up to 20 mA on both I2C ports, but you have to keep the total current in mind.
It is explained in the chapter 6.3.14 I/O port characteristics:



Instead of lowering the pull-up resistors, you could also try if you can somehow lower the bus capacitance.

What do you mean with "the square is not good enough"? I2C still works with non-perfect square wave signals. Are you sure the signal shape is the problem?
Check how non-perfect the squares are in the datasheet:



The only relevant about the signal is the I2C timings specification, given in Table 61 of the datasheet.