Can we make two winding transformer as an auto transformer...

No we cant.t hey both are designed with different configuration two winding transformer have two windings in which it provides the protection against short circuit. If short circuit occurs in one side of the winding then tranformer will not get affected but if it happens both side then only it will affect and the possiblity of occuring bith side is very less . Whereas in auto transformer only 1 winidng is there thus our circuit gets damaged by short circuit. Thus we should not do that.