Simple Two wire SPI communication between two PIC24s
You are assigning pins to peripherals incorrectly. Simply setting the bit is not enough, you must follow an unlock sequence at least once, and it is recommended to lock the register after you're done with assignment. The useful macros are contained in 'pps.h', the code will look like that (for UART2):

PPSUnLock;
PPSOutput( PPS_RP21, PPS_U2TX );
PPSInput( PPS_U2RX, PPS_RP26 );


The whole C file can be seen here -> https://github.com/felis/cdb/blob/master/fw/minimal/bsp.c#L185