What are the fundamental differences between a n-p-n type transistor and two diodes connected in series corresponding to the n-p-n transistor, apart from the metal connection in between....

can we use some other connector instead of metal to make the behaviour of the two diodes similar to the n-p-n

 

The biggest difference is the thickness of the p-layer in an NPN transistor. It is very thin for a reason.

 

can we use some other connector instead of metal to make the behaviour of the two diodes similar to the n-p-n

No the two junctions have to be physically close enough to create an electric field to sweep the charge carriers across the reverse biased junction. You can't do this by sticking two separate junctions together.