Hi

u are applying a condition on a condition. The vectors are defined in such a way that they add up by vector addition , watever be the case , even if they are at angle or the same line. But in the case of current and pressure, vector addition is not valid, if u consider the currents are in same direction then u are making a wrong assumption, coz its not the case.

u add up two currents in opp direction moving to a pt, they wont be added vectorially, they 'll just add up by simple mathematics.

U'll understand this better wen u study electric circuits , parallel resistances etc.