What platform are you on? E.g. on Mac OS there is distinct highlighting to indicate the difference:
The styles (Bold, Italic, Underline) are highlighted by coloring the glyph blue, while the mutually-exclusive alignment selection is highlighted by darkening the background. Look what a text editor on your platform does, and mimic that.
That said, I don't like the merged 'cartouche' look for the styles because it indicates a connection between the styles that doesn't really exist (and when none in the group is active, the only indicator is that none is selected, which I find a tad too subtle), so I'd probably use rectangular buttons that sit close to each other but aren't one unit to hint that they are more independent than the alignment selector.
In your case, you could have a little steam engine icon, and a little wagon icon that toggle between blue and black when clicked. At the latest when the user tries to switch over the first time, they'll notice that the old selection didn't deactivate and will click it again.