Carrie Underwood was double-dipping Thursday evening (Aug. 11) when she celebrated two of her singles—”Heartbeat” and “Church Bells”—hitting the No. 1 spot on Billboard‘s Country Airplay chart. The shindig took place at the Bell Tower in Nashville where Carrie and friends noshed on salads, chicken and steak skewers and cupcakes—from Ivy Cupcakes—arranged in a gorgeous…