volScalarField contErr1
(
    "contErr1",
    fvc::ddt(alpha1, rho1) + fvc::div(alphaRhoPhi1)
  - (fvOptions(alpha1, rho1)&rho1)
);

volScalarField contErr2
(
    "contErr2",
    fvc::ddt(alpha2, rho2) + fvc::div(alphaRhoPhi2)
  - (fvOptions(alpha2, rho2)&rho2)
);