decompose[p1_, p2_] :=
 Module[{x1 = p1, x2 = p2},
  If[x1 <= x2,
   Sum[Print[k1, x2 - x1 + 2 k2, k1], {k1, 0, x1}, {k2, 0, x1 - k1}],
   Print["Wrong values for the p1 and the p2"]]]

decompose[2, 2]

6 Null

There are 6 channels

000

020

040

101

121

202