No you do not need 2392 to be allowed to learn on 2391, but if you look at C&G web site the amount of candidate who pass the 2391 seems to be falling, some exam sitting the number how pass are less than 50%, when it first came out the the pass rate was in the high 80% even 90%, so why are less passing.
Is it getting harder, personnelly think not, what with study guides, text book, past papers etc, it should be easier.
May be it because of the experience of the candidate now taking the course, what experience of testing before the course do they possess, my friend whose lectures on these course is including more and more practical session which increases the course cost, because he finding more candidate have none or very little practical live testing experience.
This is why I believe the 2392 has come about, he ran it for the first time last year and students loved it, all who passed should go on tohopefully do the 2391 course, let see if it make a differnce.
Yes other may be using the 2392 to get extra funds.