I don't mind the arguing. It's the physical beatings taking place that freak me out. They could do some serious damage to eachother. At least with older kids you can reason with them (a little) or threaten stuff. At 18 months they don't get it at all.
We did have a moment of inspiration though the other night. Vivi noticed that Georgia had a paci and instead of ripping it out of her mouth, she went to the drawer and got herself one. Wow!