I don't think you can do this. The grid can only have one active datasource at a time.
To implement that behaviour, you need to have a datasource that encapsulates the other column (like an n-tier environment), integrating it into the datasouce the grid will use.
Now, I suppose it would be possible to have a calculated (unbound) column that could grab data from elsewhere (like a converter)...don't know if that methodology would work for you though.
Peace, Love, and Light,
/s/ Jon C. Munson II