Debugging another board using an LPCXpresso board's LPC-Link
As delivered, the LPC-Link portion of an LPCXpresso board is normally configured such that the LPC-Link (the built in debug probe) is connected directly to the MCU portion of the LPCXpresso board.
In order to debug the MCU on another board, it is necessary to make some minor modifications to the LPCXpresso board, such that the LPC-Link is no longer directly connected to the onboard MCU. NXP have a video on YouTube that shows how this can be accomplished...
and Embedded Artists (who produce the LPCXpresso boards) also have an FAQ and associated document...
There are also quite a few threads in the LPCXPresso forum on splitting the board which you may wish to peruse - for example:
You can find quite a few more by doing a search for "LPC-Link split" or similar using the search link at:
If you do not feel confident making such alterations to your LPCXpresso board, then Embedded Artists do have a version that comes preconfigured to allow use in debugging an external board. For more details please see LPC1227 LPCXpresso LPC-Link enhanced board.
Finally, if you are designing the board to be debugged, make sure that you get the debug/reset circuitry correct. For more details see the FAQ SWD / JTAG Connectors and Pinout.