I'm trying to build my own 2 way speakers. Do the woofer and the tweeter need to be the same impedance? Or can the woofer be 4 ohms and the tweeter be 8 ohms?
If it is not mandatory but does offer a benefit, how significant is it?
No they don't, but the crossover will have to compensate somehow and present a sensible impedance to the amplifier. It can be very significant in the hands of a master.
Search some of the designs and see how other people did it.