One language or two?

I think I already know the answer to my question, but I'm going to run it
by you all anyway.

One of my conlangs has a speaker base with a communal history stretching
back somewhere between 1500 and 3000 years.  (Sorry not to have a more
accurate figure.  I know one of those numbers is double the other
one.)  These people live over a distributed geographical area in
independent villages and small towns with plenty of room in between.  They
have had no written language (though that is about to change), but they
have a very strong shared oral tradition consisting of their history as a
people (which is still being composed) and their civic and religious
rites.  This oral corpus is very large, and it has the effect of binding
them together into one culture so that they have remained a single people
rather than drifting apart into many tribes with many languages.

My question is this: considering the time scale involved, do these people
speak one language or two?  I am thinking that a good deal of linguistic
change would have had to have occurred during the millennium and a half
since their first poems were written.  Would this change be of such a
degree that it would be necessary for these people to maintain two dialects
of their language, one for everyday speech, and an older, largely
fossilized, form used for the composition of poetry and for ceremonial
occasions?  How different would these two dialects be likely to be?

Thanks for your opinions.

Isidora

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