Isolating and instrumentation Amplifiers, does the order matter?
In general, when amplifying small signals, you but the main gain stage first to get the signal up to a usable level. This makes the system more noise immune, and also (if the source is high impedance) performs impedance conversion.
In this case, reversing the order would mean connecting the source (looks like the human body in this case) directly to the isolation amp, which probably wouldn't work at all. If the preamp here is amplifying electrodes on the human body, the preamp needs to be first.