A transistor is not two diodes back-to-back. It may be literally true that a transistor is two P-N junctions, but if you think of a transistor this way, you'll never understand them. The magic of the transistor is that if the P-N junctions are close enough together, you get transconductance -- when either junction is forward-biased, current can flow between the collector and emitter. Diodes don't do this!