Emulator state save files can not be used in another emu by simply renaming them.
N64 native game save files could be rename and reuse in other emus, at least among PJ64, 1964, Apollo and TR64.
For emulator state save file, 1964 has menu to import and export PJ64 state save files. To use it, you need to turn on the advanced user option in the preference. It is under the CPU menu.