I wouldn't sweat it. There may not be a reason for the bass to tune down at all, unless you come up with a great bass line that just can't live without the Low D in the song, and also complements the Dropped 'D' tuned guitar. If not done well you may actually step all over the guitar who is droning that low D at very least, - and may be droning more open strings (TONS of open tunings).
I play guitar as well (okay, okay) and spend most of my time on that thing in open tunings (DADGAD is HUGE with me) - but when recording bass to my acoustic guitar work, I've never found a great reason to drop my E to D unless I want a better way to hit more relative chord (or scale ) notes (in this case Low D). And,,, I have a Hipshot tuner on my Reggie Hamilton! Go Figure.
Identifying the guitar chords used (structure) to create the tune is more important to the bass player to deal with. You just ID the Arpeggios of the
chords and then play the relative notes, and with a good groove and you can create a killer bass line, well to my way of thinking anyway. But - I'd actually like to hear more ideas! Ya never know!