Long nose has nothing to do with aerodynamic stablity . The dual seat Mirage has come in shapes and sizes, all depending upon the users requirments.
Take a look at the pics below. It might clear the things for you.
Basically our Mirage DPs are based on Mirage 5 hence longer nose.
Does the PAF employ any Mirage IIIB's?
The aussies had them...and since we got their mirages I assume we got them too...but haven't seen a pic of them at all...?