Area of all the pipes is same A,
Velocity for pipe 1 is v1 for pipe 2 is v2, following discharge equation v1*A +v2*A =V3*A
net velocity for single pipe v3=v1+v2. In this case flow of momentum will not be conserved as rho*A*v1^2 +rho*A*v2^2 not=rho*A*(v1+v2)^2.