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-HC−H bonds we get 2xxH^+2×H+, and {C-=C}^(2-){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+1 charge on hydrogen, and a formal -1−1 charge of carbon.
In fact, the acetylide unit {C-=C}^(2-){C≡C}2− occurs as calcium carbide, CaC_2CaC2, which is an important industrial feedstock.
More reduced forms of carbon include ethylene, H_2C=CH_2H2C=CH2, C^(-II)C−II, and the methylene unit of a carbon , -CH_2−CH2, C^(-II)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-CC−C bond in such a process, we conceive we get 2xxC*2×C⋅, i.e. neutral carbon radicals.
