multiple Nios II communicate

SOPC builder already supports connecting multiple masters to the same slave, so it will work at the hardware level.

If the communication is simple then you can use a RAM (onchip or off-chip) to pass messages between the processors. You need to be careful about races between software on different processors accessing the same data at the same time. One safe way to make this work is to set up a global variable so that one processor only writes to it, and the other only reads from it.

If you want something more complex then there will be a mailbox component in 5.0 which you can use to communicate between processors.

But until then, you can use a mutex component to protect your RAM if you need to perform more complex access patterns.