From the user Kunkin KP184 manual:
"Set up online mode. KP182 load provides a multimachine online mode. The multimachine is connected through 485 communication wire. There is one master, and the other devices are slaves. When setting up parameter on the host panel and operating ON/OFF, these operations are sent to the slave through communication port and the slave will execute the same operation."
The OnLine Host/Slave mode is just a way of blindly broadcasting the Host panel key strokes to all the Slaves in RS-485 network. The manufacturer intent here was to be able to perform the same test on several different KP184 units at the same time. When this OnLine mode in engaged I think there is no feedback from the slave units or the Master for that matter (I am guessing here). So there is no way of making remote data acquisition on this mode.
This multi device mode would have to be emulated with the normal ModBus comms. The software commands would have to be broadcasted for each previously configured slave addresses, and each slave would be pooled in succession to retrieved it's data. The sample period would take a hit. The KP184 firmware is a bit slow to respond. A minimum practical value of 250ms is standard for one unit. Using 4 units in a RS485 network the minimum sample period would be 1 second. The savings here would be an USB to serial adapter per KP184.
The KP184 ModBus software as is can't do what you want via RS-485. You can however launch unlimited instances of the software on the PC communicating simultaneously with USB to serial adapters with different KP184 units which are wired in parallel with the same load and control (no need to touch the operating panel) and record the data acquisition of each instance. This method would leave the minimum sample period unharmed.
If each single unit works well via RS-485 with the software I could change the software to accommodate this feature if there is a broad interest about it. However this seems to me to be a niche within a niche.
The GUI interface would consume most of the work and comms would be the easy part, i have the same threaded low level code working with 30 simultaneous serial channels within the same application.