hiya,
i have had similar trouble with my 7 yr old, off and on and i think she has grown out of it now,
must be more difficult with 2 as they will stick together and in a sence its 2 v 1 maybe separate them when you tell them off see if one backs down without the other around
but a few things that worked for me are...
a chart, a naughty chart, people say you shouldnt do a naughty chart just a good one, but it worked for me, basically i started giving my daughter pocket money, mainly she had to clean her teeth twice a day to get full pocket money but also she had to be good, if she was naughty she would get a cross on the chart and for so many crosses she would loose some of her pocket money, also we made a right fuss over her if she was good, even if it was something small and she soon cottoned on and like that attention better then being told off, we would give her a treat or 20p something like that and just give her loads of cuddles and kisses and tell her how proud we are of her, and she soon stopped being cheeky, she also noticed a change in the whole house, the atmosphere, when she was good, everyone was a lot happier etc,
good luck