Yeah, that FM is either/or for analog/digital. Bummer.
I've done interrupt programming on a ML1400 for improved accuracy, but in the end, I have to tell clients that don't have dual output flowmeters... "use the totalizer as a best assumption"
I interrupted the CPU every 1,000ms (1sec) to take a reading, and totalized it over 60sec, then added the 1min totalized value to the 'master' retentive totalizer after each elapsed 60seconds.
I'm not familiar with the newer Micros yet, using CCW, so bear with me here. In Logix 500, I use the "STI" Function File under Controller Properties. PFN is the ladder section for the per-second totalizers, UIE, AS and TIE are set to "1" and SPM is set to "1000" (1000 msec)
Below is code, very similar to your own code at the top of this thread. The main difference is I use the Interrupt capability of the CPU.