Converting a three-wire system's "EMC filtering configuration" to a two-wire one
You need to be VERY sure you truly only have a 2wire system (CHASSIS + PWR) and not a 3 connection system (CHASSIS + P_PWR + N_PWR) and some system engineer hasn't treated a chassis reference as the P_PWR reference/return.

Consider the setup  below. 






simulate this circuit &ndash; Schematic created using CircuitLab

You as a consumer have been provided with a +-PWR and Chassis but it can be seen that the SOURCE has tied the -PWR to chassis... Why not just use chassis as the return and thus a 2+Chassis becomes a 2wire system?

This is wrong because of a phenomenon called Electromigration. The +PWR and -PWR will consist of some wire gauge with a level of impedance. If a consumer utilises chassis as the PWR return there will be a DC current flowing along chassis, as the prefered route. 

NOTE: the use of isolated DCDC converters that are then referenced to chassis is fine as the current loop will not be via chassis