You mean from the same computer? Not very big. I used to do it all the time, and trade between my accounts at the same time, stupid jagex, never got in trouble for it.
You can't get in trouble for using two computers, just say you and another sibling are playing, not just you. They're not going to ban you and your sibling for having the same internet provider, and playing from the same house, are they? They can also tell if you're using two different computers, so it's verrrry unlikely to get banned JUST from double logging with multiple computers at the same house adress.