I'm tandem-nursing my 2nd and 3rd children (as I did with my 1st and 2nd as well) and I know that feeling of needing to stop nursing IMMEDIATELY all too well. Oddly enough I've only felt it with my older tandem-nursing child... never with the new baby.
Anyhow, I keep a finger close to my dd's mouth and if she doesn't unlatch herself at the count of three, then I unlatch her. I began counting to three with the soon-to-be-older sibling during my pregnancies because I also had that MUST STOP NURSING NOW feeling during those. They usually figure it out quickly, they have some warning, and if they don't stop nursing, I break the suction with my finger and put the "naynay" away. If this upsets her then I comfort her by hugging and talking to her or reading a favorite book.
I hope you're able to find something that works for you!