That feature doesn't exist currently on the Telus STBs.
From what I read in the Xbox thread, when you press the LAST button it gives you the option of switching to any of the last 5 channels. This feature should also be available in a future STB update.
MTS has also implemented a mosaic feature in Mediaroom which allows you to choose any of your channels to include in a mosaic and easily switch between them while watching several channels at once. If Telus developed a similar feature, that would also do what you want.