The phrase 'the demarcation between' is correct and usable in written English.
You can use this phrase to refer to a boundary or distinction between two things. For example, "The demarcation between fact and fiction is often blurred in literary works."
The demarcation between primary and secondary sellers can blur.
The demarcation between serious music fans and hedonist clubbers melted away.
On the historical frontier, the demarcation between lawman and outlaw was always rather blurred.
In the troposphere, the demarcation between polar air and warmer tropical atmosphere is usually defined by the polar front.
He knew how easy it would be to float "balloon boy" when the demarcation between truth and fiction has been obliterated.
Pastoral care for medical students can be arbitrary when the demarcation between medical staff with disciplinary roles and those with support roles is blurred.
As urban lighting improved, the demarcation between city dwellers and country dwellers — still tethered to the rhythm of sun and moon — also grew starker.
The "Wallace Line", first noticed by Alfred Wallace in the 1850s, is the demarcation between the faunas of Asia and Australasia, which are quite distinct.
Ms. Kruglanski added, "I think it's meant to be breaking the demarcation between music and art, but it's hard to react emotionally".
(You may think the latter a rather high figure but bear in mind the demarcation between a KL and a camp intended purely as a killing machine, such as Treblinka).
You can only write books that are of interest to them" – he not only refuses to accept the demarcation between children's books and adult books, but also between books of any kind.
