#topDoorsLineAndPieChartsDiv {
   width: 950px;
   /*border: 1px solid black;*/
   overflow: hidden; /* will contain if #first is longer than #second */
}
#topDoorsLineDiv {
   width: 500px;
   height: 500px;
   float: left; /* add this */
   /*border: 1px solid red;*/
}
#topDoorsPieDiv {
   width: 500px;
   height: 500px;
   /*border: 1px solid green;*/
   overflow: hidden; /* if you don't want #second to wrap below #first */
}

#topUsersLineAndPieChartsDiv {
   width: 1000px;
   /*border: 1px solid black;*/
   overflow: hidden; /* will contain if #first is longer than #second */
}
#topUsersLineDiv {
   width: 500px;
   height: 500px;
   float: left; /* add this */
   /*border: 1px solid red;*/
}
#topUsersPieDiv {
   width: 500px;
   height: 500px;
   /*border: 1px solid green;*/
   overflow: hidden; /* if you don't want #second to wrap below #first */
}