It would depend a lot on the internal size of the chimney, especially at the top, although the second hole suggests that it was used with two fires in the past. If you are using the internal flue piping (approx 9" dia) on the bottom one already and it runs to the top of the chimney, provided there is enough space left to fit the second one, it should be ok....it all comes down to that internal dimension.
You will definately need the CO sensor/alarm upstairs to be working properly.