I have a 8 yr old dobie x rescue and a 2 yr old whippet/bedlington x.  we got the younger dog once older dog was very settled and well-behaved. (she had been a real handful when we got her from the rescue centre).  have had the new dog now for 19mths and they get on great (walked younger one on own for a while to get his confidence up as he was / still a bit nervy). have been to recent dog training with both together including some agility off the lead. well behaved.  BUT now, when walking them together have had a number of terrible encounters- they just go wild when they see other dogs, wanting to chase and attack- leaping around on the lead (younger one gets so excited he nips older dog and me) and this week had one incident where younger one dashed over to another dog when off lead (older one I had got on lead in time) - was fine with the other dog until i got close then went crazy - seemed to take its cue from older dog- who got very excited- and nipped and chased and harried the other dog. couldn't get hold of him and almost lost control of both.  luckily other owner very understanding- very embarrassing.  thought this was one off (how stupid am I?) and walked them into town today- every dog they encountered they went crazy over- I spent a lot of time crossing roads,etc! they never used to be this bad (and older dog is reverting to her badly behaved younger self!)- what's going on???? having read some other posts and replies I wonder if I am going to end up having to walk them individually again... but if anyone has any other ideas I would really appreciate it.  Help!?

its just 'pack' behaviour. mine are the same when road walked together. we walk them seperately and have no probs. off lead at park they are fine.

I think you are going to have to go back to basics - one at a time - you certainly aren't enjoying the situation, and neither are other dog walkers and their dogs.  

Back to basics, walk one dog at a time, you can use the walk as a training time as well - good luck!

Mine are exactly the same, it's one at a time for me too.

I agree with the back to basics viewpoint.  You can't sort it whilst you've got the 2 out together.  Sort them individually then reintroduce the walking together, time consuming and a pain, I kknow.  You said when you go to training class you take them both, does this mean you work both together in the class?  If so, don't.  Take training as an opportunity for a much valued one to one time with that particular dog, specially with the agility where dogs get excited anyway when they see other dogs working - and why not?!  It's really important, no matter how many dogs you have, to give them one to one, and it is the only way you can train them.
I'm sure you'll get it sorted and will be walking them both again with no problems.

take them to training classes at different times, it is far to distracting to take them together and they need to look to you for how to behave not each other.
walk them different times also, i would not walk or train together until you have complete control of them on their own. this can be sorted out i'm sure but you will need to be patient and put in a lot of time, worth it in the end though.

thanks everyone for all the help- i have already taken it on and started walking them on their own- its much better!  feel much more confident and they are calmer and like different dogs when alone with me- actually paying attention! and a lot less confrontational with other dogs.  Will also try the idea of taking them training on their own rather than as a pair- am looking into doing further agility and this could be something i just do with the younger one. thanks for the support- made me feel like i hadn't lost it entirely!!