how to add a right-click entry to Nautilus when 2 files are selected?

[HamletDRC]

I'd like to add a right-click popup menu entry to Nautilus when two files are selected (no more, no less) and the user right clicks.

Is this possible without installing a script add-on of some sort?

I just want to invoke a command like

diffx $1 $2

THanks,