Why would you want to?An Arduino can only do one thing at a time....R
My board now installed with RCSwitchFirmata, it can handle RC signals. I want it to handle other signal like those from acurite temp sensor.
Or just attach a function that calls both the functions you want to attach to it...
If you know what the currently attached function is,
Why wouldn't you?
Do you know what every library you use does? Most users here do not. A library could attach an interrupt handler, and most people wouldn't know.