I've got two B x's and I really feel your pain about trying to train them both at the same time! As others have said, it's best to try to get some alone time with each of them and then work them together…..
As for when out walking, I have the same issue...our first born is much better behaved, except when they are together in which case he just stirs up trouble with the other one. What we have found works the best is to take them down to the park on leads, and then let the better behaved one off the lead and attach his lead to the others lead (so it's now twice as long) and do a few laps of the park with one off lead and the other on a long lead. Once we've gotten them around a few times and they are both a bit more settled we let the other one off. This works brilliantly. Once they are both off, they behave fairly well and come when called. If we started off with them both off they just tend to wind each other up and end up running around like crazy animals.
As far as getting them to 'leave it' when off lead....I have had some success with this, although not always (foster rubbed his face in something very gross just the other day YUCK). I find a very stern 'leave it' works most times, however if it doesn't, I get as close to them as possible and clap my hands really loudly followed by a 'leave it' as loud as I can. I think this just shocks them and gets their attention away from whatever disgusting thing they were looking at.
Best of luck to you!!!
