President Reagan heads for Missouri and South Dakota today in an effort to tip the balance in two close races that figure prominently in the fight for control of the Senate. An airport rally--with balloons, marching bands and sky divers--and a $1,000-a-couple reception were on tap in Kansas City, where Reagan was to campaign for former two-term Gov. Christopher S. (Kit) Bond. Bond is battling Lt. Gov. Harriet Woods for the seat being vacated by Democrat Thomas F. Eagleton. In Sioux Falls, S.D., Reagan was to attend another rally and $1,000-a-couple reception for Republican Sen. James Abdnor, who faces Democratic Rep. Thomas A. Daschle.