I am also a mum to two and I know how you feel re the squabbling. Mine squabble over silly things sometimes and it does grind you down.....we are on week 5 of the hols now, there is light at the end of the tunnel so take heart.
However, like Flashy and Entwisi have said, you need to take control and show them that you will not stand for bad behaviour.
I don't often have to punish mine but if I do I find that time alone in their room does the trick.
My kids don't have tvs, computers or cd players in their rooms so they have to sit on the bed and have a think, then after about 15 mins I ask them to come back down and apologise to me then we forget about it...in fact I only usually have to threaten to send them to their rooms and they stop the behaviour.
I wouldn't stand for being shouted at....you are the parent and deserve respect. You must show that you are in control.
Other things you can do are ban the tv, computer games, chocolate etc.
Take it away and do not give it back until you have an apology and an assurance that they will not do whatever it was again.
It doesn't matter which punishment you choose to use....whatever suits you but the main thing is that once you have threatened to do something you must carry it through....don't back down because if you do this you will have lost all credibility, back down once and they won't forget it and the next time you threaten a punishment it will fall on deaf ears.