AngularCSSButtonActive.html

<button class="btn" ng-click="toggleVisible('left')"
  ng-class="{ active : isVisible('left') }">Toggle Left
</button>
<button class="btn" ng-click="toggleVisible('right')"
  ng-class="{ active : isVisible('right') }">Toggle Right
</button>

AngularCSSButtonActive.js

var rightVisible = false;
var leftVisible = false;

$scope.isVisible = function(side) {
  if(side === 'right') return rightVisible;
  if(side === 'left') return leftVisible;
}

$scope.toggleVisible = function(side) {
  if ( side === 'right' ) rightVisible = !rightVisible;
  else if ( side === 'left' ) leftVisible = !leftVisible;
}