Simcenter STAR-CCM+ How to set a single boundary as a inlet and outlet of two phases respectively

When the user wanted to set a simulation of cross-current flow conditions, the user would have to set a single boundary (e.g., top and bottom plane in the cylindrical pipe) as a outlet or inlet for both multiphase flows respectively.
By using EMP multiphase model with mass flow inlet or velocity inlet, this would be achieved simply.

As we already know that the mass flow rate inlet and velocity inlet could be contains negative values as input (similar to outlet).
Also, EMP models computes the governing equations for each input multiphase flow.

Combining above, we can set a positive value for phase A(inlet) and a negative value in the same boundary for phase B(outlet). So the corresponding boundary can work as inlet for phase A and outlet for phase B. 
If the user set the opposite boundary (top or bottom plane of cylinder) as a opposite values, the domain would work as a cross-current flow.

See the picture below.