How much brighter is the other light? Is it at all visible or does the brighter light washout the dimmer one? If it washes out, then you are just wasting batteries running it. My dinotte washes out my cateye LD600, so I just leave it off and save it for a back up.
If it is visible, I think I would run the dimmer of the two in flash mode. This would make the dimmer light at least somewhat more attention getting. I know some, if not most, may disagree with me.