How can I design a 8 to 3 priority encoder using 4 to 2 priority encoders?
I'm not sure how to give you a hint without giving you the complete answer.

But consider the difference between these two situations:


If any of the inputs to the upper encoder are asserted, then its outputs become the overall outputs, and Y2 needs to be asserted.
If none of the inputs to the upper encoder are asserted, then the outputs of the lower encoder become the overall outputs, and Y2 needs to be negated.