If it's a 289/302 your '68 should be an electrical oil gauge with the big fat sensor attached to the drivers side front of the block just above the oil pan with a 90* cap wire attached. The sensor will be screwed into a ~2" long hex shaped extension to hold it away from the block due to size of the sensor. You can either tap (drill/thread) a hole into the side of the extension or I removed the extension and ran a pressure hose to a 3 way tee and attached both sensors there. The 2nd gauge should be able to be electric or mech. as it is just taking or reading the oil pressure at it's sensor and sending it to the gauge.
The water temp is a little trickier as you will need to either tap into the water flow at a 2nd location on the intake (original is at the front just behind the Tstat. If you tried to take readings from the original or an aftermarket sensor they would have to operate on the same exact ohms for sending the signal to the gauge. If one of them operates on a different ohm range (at what ohm reading it sends 'hot' to the gauge, etc.) then one of the gauges will be off. One may read normal while the other reads hot, etc. I put a 2nd sensor in another intake hole for my 2nd mech. water temp. gauge.
'67 Coupe 289 stroked to 333, AOD, 9" w/3.50:1, PDB, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon