Hi,
From your description, I see that you want to use two LPC5569-EVK boards to implement the I2S communication.
In the SDK package, there is I2S master example

For the I2S slave, you can develop the slave I2S example based on the master example.
static i2s_config_t s_RxConfig;
I2S_TxGetDefaultConfig(&s_RxConfig);
s_TxConfig.divider = DEMO_I2S_CLOCK_DIVIDER;
s_TxConfig.masterSlave = kI2S_MasterSlaveNormalSlave; //DEMO_I2S_TX_MODE;
I2S_TxInit(I2S6, &s_RxConfig);
In the slave mode, the SCK and WS for I2S6 will be input pins, you can assign the I2S6 pin in the pin_mux.c
Hope it can help you
BR
XiangJun Rong