Answer: There are three classical solutions to solve the loading problem. First, check to see if the second controller supports a voltage input. If so, perhaps you could monitor the voltage drop across a device that already exists on the loop, e.g., the 1-5V DC input on a DCS or PLC analog input card. With this approach you may be able to solve the application without any additional costs or loading on the primary 4-20mA loop. However, you'll want to "parallel sense" the last load in the loop to keep everything referenced to a single grounding location. Otherwise, you may run into grounding issues. And, you'll want to keep your voltage wiring runs down to 50 ft. or less to avoid voltage drops and noise pickup.
Another classical solution would be to use a 4-20mA isolator with low input impedance (75 ohms or less) to retransmit the signal to the second controller. We offer economically priced loop-powered or four-wire (AC/DC powered) isolators with input impedances as low as 5 ohms. These products will give you another 600-1,800 ohms of retransmitted load capability on the output side. We also offer a splitter isolator instrument that will monitor a primary 4-20mA signal (sourced or sinking) and deliver two isolated 4-20mA outputs with load driving capabilitity of 1,000 ohms on each output. Each independent output could dirve its own controller (see figure below for a splitter).