The nRF51 DK board supports programming and debugging of external boards. To debug an external board, follow the instruction that extract from Nordic nRF51 Development Kit User Guide v1.0.
5.10 Debug output
The nRF51 DK board supports programming and debugging of external boards. To debug an external board, connect to the Debug out (P19) connector with a 10 pin cable.
When the external board is powered, the interface MCU will detect the supply voltage of the board and program/debug the target chip on external board instead of the on-board nRF51422.
Note: The voltage supported by external debugging/programming is 3.0 V.
The P20 also features a debug out connection in order to program shield mounted targets, as for the Debug out P19 header the Interface MCU will detect the supply voltage on the mounted shield and program/ debug the shield target.
If the Interface MCU detects target power on both P19 and P20 it will default to program/debug the target connected to P19