I'm not familiar with this kind of stylistic abbreviations but technically I see no reason why it couldn't be possible. Make the C bigger (set lighter style if available, for strokes' optical consistency), lower it's baseline; analogically, scale down the "18th" (heavier style) and raise it. Then set negative kerning between both, simple and quite obvious if you're working with InDesign or similar software. Here's a quick sample:

Just courious: what's the group name?