I would recommend to do it like the PROFIBUS people:



There's one master providing the mass "2M" and +5V-supply "2P5" for the RS485 bus. These supply lines are transmitted to each partner together with the A-line and B-line of RS485 bus. On the other hand, each individual partner gets powered by an isolated supply, in the schematic "M" and "P5". The isolation of signals is provided by optos, which transmit the TXD and RXD signals from the A-line and B-line. There's also an isolated RTS signal transmitted.

Take note, that there's no connection between "M" and "2M", even no "soft" grounding, means there are even no caps between them! But there's a soft grounding of "2M" and the cable shield, which makes sense of course.
The cable shield is hard wired to the metal case of each partner, which itself is connected to protection earth (PE-line, green yellow wire of mains). By this an entire Faraday cage is formed, provided by the cable shields and metallic enclosures of each partner.

This scheme allows all the many ways of groundings at each individual partner without the introducing of ground loops at the RS485 lines. The cable shields, on the other hand, will form ground loops, but this doesn't matter because of the optos and soft grounding technique.

Kai