
First thing for me would be an anti-stress supplement, for which I favour melatonin. Very effective, doesn't sedate. Then I'd be doing some training to get them sorted out.
The first thing that would involve is establishing how much and why the younger dog is barking - if it's just boredom then a good long walk before you go and leaving him with a kong to keep him busy; if it's separation anxiety, then you'll need to desensitise him to being left. The best way to do it if you can is to video him, so you can see exactly what's going on (he won't act the same way if someone else sits in so this needs to be done totally without human company). Then you can see if he is barking the whole time or just when you get back, too - I thought mine were only doing it then but a few days' recording established that they were actually barking and howling almost constantly :-(
If it is SA, I recommend "Treating Separation Anxiety" by Marlena DeMartini-Price, it's just recently come out. She's an american behaviourist who only works with SA cases and it's a very thorough book. I've been working through it with mine (about half have a problem due to recent changes upsetting everyone) and we are very nearly sorted out. I've got this weekend off so from about lunchtime today I'm having a big push to really build the time up and get it done.