Is it possible to install an aftermarket temperature gauge and wire it into the wiring for the factory one and have both of them working properly, would like to have a secondary reading, 03 Dakota 3.8 L V6, also can I install an oil pressure gauge and do the same with that and if so how?

 

Yes.

The PCM supplies the data to the gauges in the PCM, even the fuel level. You can get gauges that will read data from the PCI bus at the OBD port.
You could even get an app to read all the data on your phone.