Causing two gas streams to mix is harder than it sounds, even for turbulent flow and well-designed injection methods.  In many cases, static (motionless) mixers are used to improve gas-gas mixing.  Assuming that the two gas streams are of approximately equal flow rates and pipe flow is turbulent, a practically uniform gas stream will be reached in approximately 50 pipe diameters from the point of combination.  That distance could be reduced to perhaps 10 pipe diameters with a static mixer.  These distances are only rough estimates, since many other variables will influence specific mixing characteristics.