Ah, yeah.. In SilverStripe it's one template per controller. So to show an alternate layout, between two pages that use the same controller, you have a couple options. If the changes are pretty simple and non-structural, you can class and id the body element.
<body class="$ClassName" id="$URLSegment">
If it's more complex, then subclass the two controllers
AlternateCalendar.php
AlternateCalendar extends Calendar {}
AlternateCalendar_Controller extends Calendar_Controller {}
AlternateCalendarEvent.php
AlternateCalendarEvent extends CalendarEvent {}
AlternateCalendarEvent_Controller extends CalendarEvent_Controller {}
Of course, the naming pattern is up to you, but that will allow you to create AlternateCalendar.ss and AlternateCalendarEvent.ss. Just remember to change the page types in the Behaviour tab.
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