As a general rule, one MCU with more legs is better than two MCUs with few legs.
But on most occasions you can rearrange the pin budget to suit a Uno or Mega2560.
Adding an extra SPI slave to an existing SPI bus involves one extra pin per device.
Adding an extra I2C slave to an existing I2C bus involves no extra pins at all.
A separate MCU is appropriate for a parallel display with lots of pins e.g. 8080-16 or 8080-18 data bus.
SPI displays are easier with the main MCU.
I2C adapter for a 16x2 fits directly on the module. There is not an intelligent MCU. Just a port expander.
David.