I'd say just put both of your tags on it.

If the original manip is on the site then make that the parent. The source should be the same as the original manip's source.

The real question is: if GodofWarFan only added text, then should that manip have the caption_only tag even though NinjaW's manip doesn't?

Nah. That's nitpicking. Remember that the tag exists to point our manips where there was no visual manipping done. In this case, the visual manipping was done, just by a different manipper.