Did you know the small micontroller near the USB socket on you Arduino Uno or compatible is usable for much more than just a USB-serial converter? In fact it's a fully-featured MCU just like the ATmega328 and similar on various boards. With this knowledge you can make use of this as an Arduino-compatible and thus use both MCUs on the one board at the same time.
The benefits of doing so include offloading some work from the main MCU to the second one, or dedicating the second MCU to different types of interfaces with other hardware. And doing so isn't that diffucult due to the interesting tutorial published by NicoHood whose examples show how to emulate a USB keyboard and much more with his HoodLoader2 code.