Fluke 773 clamps around one of the conductors to read the current.
The other Fluke has to be in series with one of the 4-20 lines, requiring opening the circuit at one point and inserting both leads of the meter, like any current measuring meter.
The other Fluke probably has a voltage scale and if you know what the analog input's input resistance is (stated somewhere in the specs) the voltage (IR) drop can be measured and converted to an equivalent current with the dreaded Ohms Law.