Oh, I see
.
Then as the issue seems to be a selfish one of the Kobo's I can only suggest a USB cable with a switch as BetterRed has suggested, or it may be that a USB management application for Windows along the lines I mentioned may work but I have no knowledge as to their competence if an MTP device is attached to the computer's USB port.
Regarding switched USB cables I know they are used to switch the power conductors in the cable for when connected to a charger but it may take care to ensure buying one which switches the data lines as well, if that functionality is required by the Kobo - maybe BetterRed can elaborate - I just pull and replug cables myself.