Resources on multi-MCU architecture?
In the past I've ran into this problem, with the STM32F's, but the problem wasn't a lack of GPIO's, it was a lack of processor power. I ran two processors on the same board and used the UART's for communication.
You won't find tutorials on this, you'll have to code up your own solution. But if you can code a timer or a BLDC control loop, sending a message from one processor to another  over a UART (or SPI) shouldn't be that hard. Use a hardware UART to take the load off processor.
Also the STM32H7's have a lot more timers than the STM32F4's so you may want to look at those, use the cube mx tool to help you configure the processor or locate the one with the right resources.