If you're using electrical gauges (as you specified in your original question) in addition to indicator lights, yes you will need two different types of sending units for both, and you'll also need separate wiring for both. The lights are controlled by a simple on/off switch. Once the coolant temp passes a certain point, the temp switch turns on (shorts to ground) thus turning on the light. If oil pressure is not present, the oil pressure switch shorts to ground and turns on the oil pressure light. Electrical gauges need sending units that are more than just on/off switches. Sending units for electrical gauges "read" either the coolant temperature or the oil pressure and turn that into an electrical resistance that is "read" by the associated gauge and displayed as a value (digital) or a movment of the needle (analog).
If you're going to replace indicator lights with gauges, you -do- have to discipline yourself to develop a habit of visually scanning your gauges often enough to notice a problem before it gets serious. Pilots are accustomed to doing this.
People can and do "miss" indicator lights too, it happens all the time, but I do agree that in general they're easier to notice especially if they're big enough, bright enough, and located in your normal field of vision.
You could avoid part of the "dual sending unit" problem by running a mechanical oil pressure gauge as opposed to an electrical one. If there's such a thing as a mechanical coolant temperature gauge though, I'm not aware of it.