A transistor is a device that is used either as an amplifier or a switch.
Diodes that are connected back to back cannot function as a transistor.
A PN junction diode is supposed to have equal doping on both sides. But in the case of a transistor, the doping level for the base should be low, and that for the emitter should be high.
The merging of the two diodes will result in the base size exceeding that of the emitter and collector but for a transistor, the base size should be kept to a least. Moreover, the construction of a transistor is monolithic and therefore cannot be constructed by the merging of the two diodes.