It sounds like you have two continuous flows that you want to mix, that the fluids are miscible, and that you want to do the mixing in the pipe that is conveying the fluids. If so:
1) A pipe will not mix longitudinally, so both flows need to be accurately metered. I once had a project where we conveyed a water solution through 4800 feet of 2" pipe. There was a concentration meter at each end. Step changes in concentration at the supply end came through as identical step changes at the receiving end an hour later.
2) If you have turbulent flow (Hint: calculate the Reynolds number), then you should get transverse mixing if the pipe is long enough. You can improve transverse mixing by injecting through a perpendicular nozzle sized to get reasonable velocity, adding some elbows, or by adding a static mixer.
3) Is your system open loop (set the flows, hope for the right ratio), or closed loop (measure the mixture with a concentration meter and feed back to the supply valves / pumps? If closed loop, make sure that the fluids are fully mixed before entering the concentration meter, and that the concentration meter is as close as possible to the supply valves/pumps. The flow time from the supply to the concentration meter is a delay, and delays make for really ugly control situations.
4) How to control flow is determined by viscosity, flow rate, pressures, necessary flow accuracy, line losses, and a few other variables. We need a lot more information to make recommendations on how to control flow. Start with a flow diagram that includes all available information.