I think it is questionable to connect two gauges to one sending unit...unless the garmin unit is made specifically for this purpose.
The mechanical gauge provides a small source of voltage and current to the sending unit to measure it's resistance. If the garmin is doing the same thing, you now have two sources of voltage and current, which would not be desirable in parallel or in series. To make it more complicated, the gauge is analog, while the garmin is likely to be digital.
I would try the Garmin by itself to see what readings you get. More importantly, I would check the documentation on the Garmin to see if it can be hooked up to the same sending unit as an analog gage, and if it hooks in parallel or series.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings