Well if you see the killa kan rokkits, you see they have two even though it is just a single rokkitlauncha. To last the whole battle you would need six or seven really, but I guess the principle would be something like:
Turn 1: One rokkit fired.
Turn 2: Second rokkit fired and first one reloaded
Turn 3: first rokkit fired, second one reloaded
And on like this.
(I do not know what would be reloading it, but still.)
So with the twin rokkit launcha you infact have two Kan rokkit launchas. 2 rokkits on the rack generally means a single rokkitlauncha, 4 rokkits generally means twin linked rokkit launcha. Note that this principle just goes for vehicles usually.
When it comes to big shootas, when they are twinlinked, more ambitious converters let 1 looted assault cannon or other minigun weapon represent the twin bigshoota. Other people let two heavystubbers (who have three shoots but strength 4) represent one big shoota or looted heavy bolter. As such, The koptas twin bigshootas will sometimes be represented by four heavy stubbers. Other people say 2 shootas=1 big shoota, and represent twin bigshootas with quad shootas.
Thanks for listening
Edited to get soapbox right.