Assuming
$scope.filterToggleBtnSide = "class1";
$scope.notificationColor = "class2";
You can do something like this:
<div class="filterToggleBtn thumbnail class1"
ng-class="{class2:notificationState }"></div>
<!-- or this -->
<div class="filterToggleBtn thumbnail"
ng-class="{class1: true, class2: notificationState}"></div>
<!-- or this -->
<div class="filterToggleBtn thumbnail class1
{{notificationstate && notificationColor}}"></div>
<!-- or this -->
<div class="filterToggleBtn thumbnail class1
{{notificationstate && 'class2'}}"></div>
<!-- or this -->
<div class="filterToggleBtn thumbnail
{{filterToggleBtnSide}} {{notificationstate && notificationColor}}"></div>
<!-- or this -->
<div class="filterToggleBtn thumbnail
{{filterToggleBtnSide}} {{notificationstate && 'class2'}}"></div>