How can I add two Nios in FPGA?

Nios II has supported multiple CPUs since the first release.

Each CPU has a system library associated with it, so the system.h and autogenerated linker script apply only to that CPU.

You need to create system libraries (and application projects) for all the CPUs in your system. The appropriate memory map will be chosen depending on which memories are visible to the CPU.

You also need to ensure that the text and data sections of the different CPUs are in different memories, and that they don't share the same reset and exception addresses.