The timer is your friend! I use "Tickety" for times when I think my daughter will fight and/or argue over doing something.
So, it goes like this: "You need to clean your room. I'm going to set Tickety for 5 minutes - if I don't see a differenct, I will clean the room and I use trash bags to clean." I will usually give her one "free try" where she didn't make any progress, but I ask if she would like to try again and offer suggestions on how to make a difference.
Kids can't argue with the timer and there is no negotiation with Tickety - time stops for no man! So, sit the boys down and tell them that starting in the morning, they have 5 (or 7) minutes to get dressed. If they don't accomplish this before Tickety rings, they will ...(have to give up a toy; loose a privelege) Or you could tell them that if they aren't done before Tickety rings, you'll pick their clothes for them. Then pick some really strange thing and that is what they have to wear that day.
It will take a little work to get this going. The first few times, they won't believe you - so you've got to be prepared to follow thru on your threat!
I used to also use a "time Out Box". If she didn't get dressed before Tickety rang, I got to pick one toy to but in the Time Out Box and she had to earn it back. I ended up with 2 large boxes full of toys before she got the point! But, eventually she would earn her things back.
Whatever you try, good luck! This is (for me) the hardest part of parenting!