According to the datasheet the nRF52840 (and nRF52833) chips have a second UART available.
https://infocenter.nordicsemi.com/topic/struct_nrf52/struct/nrf52.html

Could the second UART be activated and used by adding a uart::Uarte field to Nrf52840DefaultPeripherals and adjusting the base address? The base address would be 0x40028000.
Should this work or am I missing something (Clocks, interrupts, ...)?