As you know, I am really an interloper as I don't have cockers, but so far manage all 3 on single leads. However, I am very tempted to get a double for Suki and Josie as Suki is brilliant on the lead and Josie copies her. I have Suki on the outside and Josie between me and Suki and Meg just trots along on the other side. Suki and Josie are both training to walk on the left, but as Meg wouldn't know a training class if she were dropped in the middle of it, she just walks whichever side I put her on and tries to superglue herself to my ankle!
If I lived away from cars, I wouldn't even use a lead for Meg, but you can never take the risk when there are cars about.
I will have a look for a double lead soon......