Re: One Fuel Sender, Two Gauges.
The sender in the tank is likely a leaver type resistive sender. As the float moves up and down the resistance changes.
The most common "OHM" range is 240 -33 ohms. (Emty to full) There are other ranges out there. (10-180 or 0-90)
Normally if you wish to use two simple gauges you need a sender set up for dual gauges which means the sender has half the resistance in order to accomodate the resistance in the second gauge. ( like: 120 -15 ohms)
Dual senders will usually have a "D" stamped on them or the part number for a dual unit.
That said, your multifunction gauge, if digital , may be setable for various ranges. The best thing is read the directions. Then try it. If you take out the sender you can move it up and down to test it.
Hope that helps!
Will & Muffin
Lucy the dog
"Yes, well.. perhaps some more wine" (Julia Child)