When two plane balancing in the field I always find a certain amount of fluctuation in both magnitude and phase in both planes. To get accurate readings I believe that it is best to take both balancing readings simultaneously as both planes generally fluctuate relative to one another. So why is it that I can do this with my Enpac data collector but not with a CSI2120 or CSI2130?. I understand that these data collectors can collect dual channel phase measurements on route but not in the balancing program Why not?
Does anyone have any comments on this?

I think you are right that the 2120 cannot take "simultaneous" dual-channel data in the balancing program. FastBal has not been funtionally changed in the roughly 15 years I've been using it. FastBal II is the same as the original program, just with a prettier face (doesn't add any real functionality). It was written before there was a 2-channel 2120, and so doesn't support simultaneous data collection. They do have a 4-channel adapter that allows you to take sequential data using up to 4 accelerometers.

But I'm not sure this is a problem in FastBal. It takes "average" readings, the number of averages determined by the user. I like the original FastBal because it shows you the instantaneous and average amplitude and phase readings. I think it best to take many averages when the data is fluctuating.

I was balancing a belt-driven fan yesterday that ran 5 rpm faster than the motor and there was a considerable "beat" present between the two. On each reading, I just let the meter average until the numbers stopped changing, and then hit the button to record the reading. Though not as good as usual, the final balance was acceptable.

But in answer to your question, I can't see why FastBal couldn't be fixed to take 2-channel data.

It is much better to average the readings either by hand or as Rusty did with the analyzer than to try to get simultaneous points. You are trying to establish an estimate for the steady state vibration at 1X.

Also, when you compare the data with the original and the trial runs the runs need to have the same conditions. Since the vibration fluctuates, average each run to get a good estimate to compare for balancing.

Well, if it has true 2 channel capability it´s not so special to take the 2 channel data simultaniously and average both to the liking of the user. What I like with the vintage Fastbal I use is the bar graphs that indicate the result of averaging both amplitude and phase so if you for some reason feel lik you are in a hurry (the thing is close to lift off and walk out the door) you can hit a button and accept the reading when you decide, or wait until it´s as stable as it gets. I also like the live vector plot with max and min indication so when I get down so the residual is only from bearing clearance it start to walk around all angles and there is the limit, no more to do. Olov

If the machine is 'close to lift off,' I hope you have long cables and can do startups and shutdowns, because the transient information can be very valuable. Also, if the vibration is very high, one must decide whether or not to balance. Know when to hold them and certainly know when to fold them.