There isn’t a one-button shortcut to switch the 600D from movie mode to Av mode. On this camera, changing from video to Av requires using the mode dial.

You can take a still photo while video is recording, but it won’t be shot in Av mode. The camera uses the movie recording settings for that image, and recording may pause or skip for about a second while the photo is captured. If needed, you can enable autofocus with the shutter button during recording in the menu, but autofocus noise may be clearly heard in the video.

So, if you specifically need Av-mode stills, you’ll need to switch modes manually.