For adapting two HP41 together use two HP-IL/RS232 interfaces, or alternatively two HP82166A IL-Converter units. The belonging HP82166A manual describes this special application including needed wiring and software.
But much more interesting for the HP82166A IL-Converter are advanced hardware interfacing and measurement and control applications using the HP41 : „Control the world with HP-IL“ Two nice interface systems are absolutely powerfully hardware platforms for doing this : The battery powered, modular I/O-Board and the IL2000 system.....
The best today solution for interfacing two HP41 handheld computer systems together is using the new full version of EMU41 from Jean-Francois Garnier , plus PC with HP-IL/PC Interface Card :
First connect the first HP41 to this system and upload data, programs, any type of file or non normalised numbers = NNN´s. to virtual EMU41. Than connect the second handheld and download data, ,programs and so on to the second machine.
Emu41 gives you much more features : virtual display, printer (including barcode plot), mass storage, gateway to the DOS world, and an powerfully emulator including advanced plug in modules and ram-box features. More details about EMU41 you find in the HP-Museum article forum item 311 “HP41-PC gateway &emulator”.
Best wishes from Germany – Christoph Klug