Two toddlers close in age..CONSTANT fighting!

So my title says it all.  I have a 3 & 2 year old..14months apart.  They are non stop at each other's throats..no matter who we are with or where we are at.  The oldest is a boy and youngest a girl.  I know I'm not the only Mom going through this..any advice, suggestions would help.  It's like they're in the Ultimate Fighting ring, they punch, bite, push, pull, pinch, kick anything to inflict pain on the other.  I break them up and turn my head to do something and they're right back at it!!!! I feel like pulling my hair out most of the day (I'm a SAHM with them).  They do have their good moments where they are laughing and playing together but it seems to always end in a fight for some reason.  And when they are apart or I have one at a time, it's sweet relief, they are great.  Is it just all of my attention they want??  But when they are together, which is mostly the case and I'm paying attention to one, even though they have all my attn they'll still fight!?  It's like I can't win. My 3 1/2yr old still cries and screams a lot and throw's temper tantrums and throws toys when in time-out. He just seems so angry..is this normal I wonder. But then again I know there is no norm when it comes to kids. But I also wonder if he/them will get better when he starts preschool??

Anyone going or have gone through this????