I would use the spare slot.
Then if you have a weird reading some time you can check with the other in a first line approach to what's wrong.
Occasionally it is also useful to be able to copy directly from one CD to another
You won't be overloading your PSU since neither drive will draw current once in an idle state.
Ventilation might just be a snag with more clutter present, though you can choose cable routing to minimise the heat risk.