[Kitty Kat]

Does any1 know how to complete this equation?
FeS + Cu2S + O2 -->

[AWK]

Depending on temperature and amount of oxygen you can obtain Fe2O3 and Cu2O or Fe3O4 and CuO.
Do reaction separately, eg
4FeS + 7O2 = 2Fe2O3 + 4SO2

[Donaldson Tan]

Cu₂S + 2O₂ -> 2CuO + SO₂

[Tetrahedrite]

Both of you are almost right!

2FeS + Cu2S + 7O2 -> Fe2O3 + 2CuO + 3SO3

Cuprite (Cu2O), magnetite (Fe3O4) and SO2 are unlikely to form in the presence of sufficient oxygen

[AWK]

Reaction wrote down by Tetrahedrite is only valid for 1:1 molar mixture of FeS and Cu₂S and SO₂ not SO₃ is produced in this way. To convert SO₂ to SO₃  a catalyst is needed.

[Tetrahedrite]

At high enough temperature no catalyst is required! We can safely assume a reaction such as this is not occurring at room temperature otherwise base metal mines all around the world would start catching fire! In the presence of the atmosphere in this case the thermodynamically stable species are CuO (tenorite), Fe2O3 (hematite, with time) SO2 at the earth's surface is unstable and will eventually be converted to sulfate and incorporated into the soil as a soluble salt.

[AWK]

At high enough temperature no catalyst is required!
If so, why SO3 is produced from FeS2 in two steps
4FeS2 + 11O2 = Fe2O3 + 8SO2
and
2SO2 + O2 =SO3