Matt,
I agree with Nic - the snip won't work in this case
It's all about "knowing/testing" their boundaries.
As you are well aware, the cocker is a live-wire, having two is double the "fun" but it means you (& partner) must be Alpha (& Beta) dogs AT ALL TIMES.
If you can make the time to set/reinforce the "pack rules" fair enough and remember, these breeds need lots of exercise (at least 2 *1 hour stints per day) and plenty of mental stimulation (during the day only) to stop any destructive behaviour in the house/car etc.
I say "during the day" as they need a regime which differentiates between daytime/nightime and makes them think "okay, it must be nearly bedtime cos they are doing x,y,z".
You have to reward good behaviour (with praise - not necessarily with treats) and dissuade the bad by associating it with a "consequence" such as a spray of water or a noisy rattle.
It may sound daft, but if you can give it time & energy, you may be able to turn things around; the hardest thing would be to rehome one of them - but you must consider what is best for everyone concerned.
Best wishes
Mike
"It is infinitely more exciting to live a life of catastrophic failures than a life of could-haves, should-haves & would-haves."
Moh Hon Meng