Combine 2 Unicode letters into 1 in Fontlab Studio

Being new to Fontlab Studio 5, I'm creating a Tifinagh font using its Unicode block...

I created all the letters except that I need to add a ligature (if this is the right word?) as shown below;

So when user types the sequence of the two letters; I format the two letters into 1.

I tryed the Glyph menu and Generate Glyphs but doesn't seem to work :)

Is that possible at all or I'm missing something?

Thanks!