If you used one of the 20MHz packages, like MightyCore for 1284P, clocked it 20 MHz, divided the system clock by 2 internally and then output it on the Clock Out pin, 10 MHz would not be a problem.
If you used one of the 20MHz packages, like MightyCore for 1284P, clocked it 20 MHz, divided the system clock by 2 internally and then output it on the Clock Out pin, 10 MHz would not be a problem.