First would be to isolate the ground connections between the two scanners by inserting audio isolation transformers (ground loop eliminators) in the external speaker to computer cables. This will eliminate any potential difference in ground potential between the two scanners and the sound card. The negative side of the external speaker jack on some devices is not at ground potential. In some scanners both the tip and ring are driven in opposite polarity to increase the audio power. Grounding the negative side by sharing a common connection to another scanner will often cause problems.
Bob Peloquin - KB1VUA
Uniden HomePatrol-1, BC100XLT,
R/S PRO-30, Pro-106, Pro-163 (feed provider)