Hi mariswamy,
It is recommended to complete it register address and data write in a single transaction. Assuming that setting the last parameter of the function nrf_drv_twi_tx to 1 allows for a single write sequence, the second call of the function nrf_drv_twi_tx to write the data should have the last parameter set to 0 to complete the transaction. If this doesn't work, then you may have to create a single array of len+1 and copy the register address and data accordingly.
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