I want to say that we cannot represent 23 in Roman as both IIIXX and XIIIX.
The correct representation for 23 in Roman is XXIII.
If I write like this
XXIII, neither IIIXX nor XIIIX, represents 23 in roman.
is it correct grammar?
I would say that it is probably grammatical, but because ‘neither’ is not common introducing a clause like that, it is very confusing: it is not clear on first reading what the relationship is between the main clause and the parenthetical phrase.
Apart from the suggestions already made, ‘but’ would mend it, as it makes the relationship between them clearer:
XXIII, but neither IIIXX nor XIIIX,