20 (SDA), and 21 (SCL) pins for the I2C bus and interrupts

Projects Networking, Protocols, and Devices
1

The 20 (SDA) and 21 (SCL) pins in my Arduino Mega is already being used as interrupts for an encoder. Now I want connect an Arduino Uno as a slave through I2C so is it possible to use the same pins?

Or else can I define another two pins as SDA and SCL?

2

Only SCL/SDA can be used for I2C, as there is hardware within the chip to support it.
Can you move the encoder pins elsewhere, to a couple of pins with PCINT support?