I first want to say the spankings for your boys were well needed and deserved. In my home, if the kids continue to argue I usually first try to let them work it out. But I've told them if they can't do so, or if it has especially gotten to the point of physical fighting, then I will resolve it and they will not like how I do so. Which has always meant spankings all around.
As to your thread topic, if I am spanking more than one child I have always done the same as you and have bared all the children before taking any of them over my lap. I will remove everything from the waist down (excluding socks, if they are on) and give my scold/lecture session including making it clear to all of them why I am spanking.
And then, one at a time, I will turn them over my lap. If the misbehavior is equal between all of the children I will go from youngest to oldest as I know waiting for the older children is a bit worse for them. Where I don't spank the older children any more serious than normal if the misbehavior is the same (with the exception of differentiation due to age), I feel with the children being older should have set a better example instead of showing what "shouldn't" have been done.