Note that acetylene is neutral and while we can speak of the oxidation numbers of its atoms, we cannot speak of the molecule's oxidation state.
If we break up the C-H bonds we get 2xxH^+, and {C-=C}^(2-) (carbon is more electronegative than hydrogen, so when you (for the purposes of assigning oxidation number) break this bond you put a formal +1 charge on hydrogen, and a formal -1 charge of carbon.
In fact, the acetylide unit {C-=C}^(2-) occurs as calcium carbide, CaC_2, which is an important industrial feedstock.
More reduced forms of carbon include ethylene, H_2C=CH_2, C^(-II), and the methylene unit of a carbon , -CH_2, C^(-II). Oxidation state assignments are of course formalisms; they do not have real significance other than what we assign for them. When we break a C-C bond in such a process, we conceive we get 2xxC*, i.e. neutral carbon radicals.