How to set values simultaneously with flowcontrollers

[Hologram]

I have a program with settinig the flow for 2 flow controllers. Each is programmed so you have to set them individually. My problem is that I would like to have a constant total flow as these tubes are connected together after flowcontroller. If I change flow controller nr 1 I have to enter that value changed flow speed before I am able to change flow controll nr 2 back so that same total flow is the same. But this time between will give a bump in my measurement due pressure change. 

I just want to be able to change the flowcontroll speeds on flow nr 1 and nr 2 simulataneously so that the total flow is the same during this change of concentration in the gas. 

Any ideas how I can modify the program to do this?

[Dennis_Knutson]

Where's your program? What sort of connections do you have to the devices? Are you using parallel loops or reentrant subVIs?

[natasftw]

It sounds like you're going to want to look at this idea: http://en.wikipedia.org/wiki/PID_controller and work with parallel calls.  But, if you answer Dennis we can help you a lot easier.

[chembo]

I guess, beside LabVIEW, there are many other important factors in this case.

What is the type of your flow controllers? Pressure based, thermal, etc... Flow controllers have different response times depending on type and also communication type (analog, digital). If the response time is slow, there will be probably not a big difference between simultaneous flow change, or flow change with 20 milliseconds offset.

I think also that the correlation between the flow and the flow change has bigger impact, i.e. 1% vs 10% change.