Thanks to you for the extremely quick response and fix, EmfDev.
Yesterday after going through the airport security, the total uSv was slowly increasing (without any counts measured) and suddenly it dropped a big amount and continued increasing slowly. I have pictures and a video! That made no sense for an integral of the dose rate during the whole period. Now the calculation seems to be right: TotalCounts*sensitivity/60, and it stays unchanged until there is another count. I cannot imagine what you were calculating before!
I'll also check the algorithm when I install the M4011 tube again.
I think there is a circuit that generates a low when there is a current in the tube. Each tube has one. It's probably that discharge in the capacitor is detected then generates a low if the charge goes below a threshold. But I need to confirm this.
So if I wire the LND 712 to the positive ahead of the anode resistor on the SI3BG and its ground, with the M4011 turn off with hardware switch, the counts will most likely be coming from the LND which is much more sensitive than the SI3BG.
Do you mean to connect it in parallel to the SI3BG tube? Yes it can work. As long as it's in the working voltage of the two tubes. And yea, the count will more likely to come from the more sensitive tube.
Another question. When ZLM used the LND 7317 tube with the 500+ power supply he cranked up the voltage to the PS. It is true that the PS in the 500+ is essentially the same build/components as in the 600 and 600+ (except for the separation hardware to feed two tubes? I think ZLM said the PS could support 600V by design.