Most ogham letters, forfeda included, were built by assigning a new stroke pattern to a needed sound. Emancholl was built differently, by taking an existing letter's form and doubling it, and naming the result for that doubling rather than for a tree, a loanword or an abstract description. That choice is a small but genuine window into how the scribes who extended ogham thought about the system they had inherited: not only as a fixed stock of shapes to memorize, but as a productive convention they could vary to generate a new, related sign when the situation called for one.
Whether Emancholl saw any real use outside the grammatical tracts that preserve it is not something the surviving evidence settles. None of the roughly four hundred inscribed ogham stones carries a forfeda letter of any kind, and Emancholl in particular, doubling an already rare need, ch, onto an already late invention, sits at the thin edge of what can be called an attested part of the alphabet rather than a scholarly proposal for how one might have been built. That thinness is worth stating directly rather than smoothing over, because popular ogham material tends to present all twenty five letters, core and forfeda alike, as equally solid, when the evidentiary weight behind each is genuinely uneven.
Sources A Guide to OgamDamian McManus with Auraicept na n-Eces: The Scholars' PrimerGeorge Calder (ed.)