Twins biting!

my girls starting doing this around 14 months. I have read lots of info that says it's really common for twins to do this to each other but they generally don't do it to other children.
Unfortunately, mine are STILL doing it and they are nearly 2. I put mine in time out if they bite, pinch, push, pull hair etc. They don't do it to anyone else, just each other. Time out is in the cot which is easy now since they are sleeping in beds and I just left a cot assembled in the spare room. But when they were still sleeping in cots, I still used one for time out, I would just leave the door open (they always have it shut for sleeping) and make sure I explained to them that they were going to time out for biting. When they get out, they have to give their sister a hug and now tht they are talking, I get them to say sorry as well.
When I am consistent, it really works and the biting did decrease from ALL the time to very rarely when I jumped on it with time out straight away. It is harder now because they retaliate and by the time I get to them, I have no idea who started or who deserves time out so I am finding the biting etc is increasing again because I am not being as consistent.
Looks like I'm going to have to strat being stricter with it again. *sigh*
It's hard work but it does work! good luck!