There's only one GO route into KW with any real service, route 25 to Square One in Mississauga. It's not hard to catch a double decker on it, they're far more common than the single deckers. The exception is the 25A, which goes to Milton and I don't think has double deckers.
If you're just riding it to try a double decker you can catch it in Waterloo at UW DC and take it to Kitchener or Cambridge, then catch the iXpress back. Be aware most letter versions run express, so a Waterloo to Cambridge trip requires a plain 25.
If you're just riding it to try a double decker you can catch it in Waterloo at UW DC and take it to Kitchener or Cambridge, then catch the iXpress back. Be aware most letter versions run express, so a Waterloo to Cambridge trip requires a plain 25.