My dog has started attacking my other dog when they both start barking

My advice is to not allow them to get to this point (over the threshold). If you can see they are going to start barking like mad at something, remove them BEFORE it starts if at all possible. At the very least, separate them quickly and safely so they can't fight.

They feed off each other's energy and if one has anxiety or fear issues, she is likely re-directing to something she CAN get at (as Lilith said), your other dog.

I have two large dogs; one is fearful of strangers. This started happening to us when I would take them out for walks. If another person would approach (even across the street), my dogs would both start lunging and barking. One was excited and wanted to go say hi, and the other was terrified and was trying to scare off the intruder. At first I could just keep walking and once the person was far enough away the dogs would calm down. Unfortunately it got worse and eventually the fearful dog would turn to my other dog and attack her. It wasn't personal. It's almost as if he loses his mind so much and is so focused on the person that he doesn't see her at all, then suddenly he sees another dog directly beside him barking and lunging so he attacks. This usually only lasted several seconds, but it's a scary several seconds. To be honest, I've stopped taking them for walks together because of this. They are too large for me to properly control/manage when they are in state and so the behaviour went unchecked. We have since spent a lot of time working on his fear issues, but I still don't walk them together anymore.

Now, they will do the same thing if anyone comes to the front door, particularly the mailman. They're both barking and freaking out at the window that's right beside the door and if it goes on long enough, they will fight. It is also a small space, so they don't have room to get away from each other even if they wanted to. This always makes a fight more likely. So I try to keep that blind closed so they can't be so close to the problem (mailman). Also, if I know or see in advance that someone is coming to the door I will distract the dogs and get them away from the area before things escalate.

The bottom line is, you need to prevent it from getting to this point with your dogs. Figure out what exactly the triggers are and know at what point they become over threshold. Then work on avoiding the triggers altogether and if that's not possible, intervene BEFORE they escalate.

It depends on what they're barking at too. It might be as simple as closing the blinds, or blocking them from seeing out the window some other way, or you might have to keep your dogs separated (use baby gates, etc.). Also look up the safe way to break up a dog fight (wheelbarrel method). If it only lasts a few seconds, like it does with my dogs, throwing a glass of water on them works for me. Usually I can't even get that far before it's over though.

Also work on the separation anxiety and anxiety in general in the dog. There's a great little book by Patrical McConnell called "The Cautious Canine" you might want to start with. Make sure you only use positive-based training methods.