Question: I have a schottky diode in a sot23 package that is composed by 2 schottky diodes with a common catode connection. I need to use one of them.

Should I let the second floating with unconnected pin or should I connect it to ground?

Answer: 
If the common is connected to a low-impedance node you can leave it floating. If not, I'd short it out lest it act as an accidental antenna/detector. 

I'd also suggest using the single out of the pair that would be present if you bought the one-diode version of the 3-pin package (if possible), just for future compatibility. IOW if you're using the [BAT54C](http://www.mccsemi.com/up_pdf/BAT54-BAT54S(SOT-23).pdf) or BAT54S, use pins 1 and 3 and either leave 2 open or short it to 3. If you're using the BAT54A you can't do this. 

![http://www.mccsemi.com/up_pdf/BAT54-BAT54S(SOT-23).pdf](https://i.stack.imgur.com/YJ5SQ.png)
If it is literally just a 2 diode package the extra diode can float with no ill effects. Any other connection may cause problems.