Question

Can two pn junction diodes be placed back to back to form pnp transistor

Answer 1

yes, thats right by joining two PN back to back will result to be a transistor and it has posibility of forming "PNP" or "NPN".
If PN & NP are connected back to back it forms PNP transistor, if NP & PN are connected it forms NPN transistor.

There is another case when we connect PN & PN diodes we might possibilt obtain a rectifer or a sort of rectifier.