To reproduce:
-add RadSplitContainer (horizontal) and add one left and one right split panel
-add two RadButtons to collapse and show one of the panels
private void radButton1_Click(object sender, EventArgs e)
{
    this.splitPanel1.Collapsed = true;
}

private void radButton2_Click(object sender, EventArgs e)
{          
    this.splitPanel1.Visible = true;
}
-click the first button to collapse the left panel; as a result the second panel fills the entire container size
-click the second button to show again the left panel; as a result the panels overlap each other