This is a common question that is asked from plant personnel especially where a DCS system and a PLC may function together with a transmitter signal being common from the field.
Putting the DCS and PLC in series into the current loop is the logical solution but there are some drawbacks or limitations. Often the PLC and DCS does not have isolated individual channels and you are sharing the input card with other devices from the plant. The volt drop across each input could be as much as 10V (500 Ohms) or at best 5V (250 Ohms) and then there may be other devices in the current loop, you very quickly run out of loop voltage which is typically 24V dc.
A quick fix is to install a Loop Splitter (Model No C2464B) One 4-20mA input and Two 4-20mA repeated outputs fully isolated from the field and from each other.
Link to PDF https://www.omniflex.com/pub/downloads/DSC2463B.pdf
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