You can push them after you apply bindings like this:
this.selectedOptions = ko.observableArray([]);
//The single selected option
this.selectedOption = ko.observable(selected1);
var vm = new ViewModel()
ko.applyBindings(vm);
var selected1 = vm.groups()[1].children()[1];
var selected2 = vm.groups()[1].children()[0];
vm.selectedOptions.push(selected1);
vm.selectedOptions.push(selected2);