Another issue was the fact that the synchrometer was fed by the HPRPM Tacho-Genny, so the pilots would line up all their rpm's to stabilise the synchrometer indicators, then the eng would start fiddling with the throttles to line up his LP rpm's on his gauges, knocking the synchrometers out again, hahaha.
The same was true for "throttle stagger" snags - eng would line them up on LPRPM, we would ground run and line up on HPRPM (as per the AP) and pronounce it No Fault Found, lol.