I would like to power the VIM3 using a 12V to 5V DC-DC converter and the 4 pin using the Molex connector. I would like to know if it is possible to use a wire splitter or a terminal block to produce the two 5V (+ves) from a single 5V output and combine the two GND pins to a single GND pin.
Assume they mean that the want to take one 5v supply and split that line to run to each of the two 5v connections and same for the 0v lines. If that is the case then yes that works as the two power and two ground pins in the vin connector are joined together on the vim3 anyway. But might as well ditch the 12 to 5v DC to DC converter as the vim3 can be powered from 12v (actually recommended) anyway