Thanks a lot for the quick reply guys,

Some time ago to solve this I tried to use virtual COM ports, in particular with com0com and hub4com), but sometimes on concurrent accesses the whole thing hanged, the serial port seemed to become sort of stalled, and I had to reboot Windows.

Do you know of any reliable alternative? Once I implemented the solution kuzulis proposed, for a particular problem, but this time I need the solution to be more transparent to the regular serial port access, so either I use virtual ports (which are no more than kuzulis' solution but with virtual devices involved), or I try to only open the port when I use it and close it right away afterwards (dirty, but implemented it once and worked more or less nicely).