I am sure this is a very subjective topic...those of you who might use two acoustic guitars for a gig, one standard and one alternative tuning. I am trying to learn more of what is your go-to alternate tuning, and some reasons why that's your choice. Thanks

Whilst i’m an electric player ....... To get richer sounds and nuances which would be the same for you being accoustic.

For me its a question of adpting to the tune in question.

Open G for certain tunes, open E for others, or even open D if you don’t want to stress your guitar.

Jerry

I used open E for slide when I gigged in a blues rock band. Don't gig any more, but I have a 2nd acoustic tuned to DADGAD which is nice to use when I want to get away from muscle memory habits.

I used to fiddle around with DADGAD when I specialized in Celtic/folk pieces, and I have been known to use DADGBE and EADGAE at times (especially for banjo style frailing), but in truth my inherent laziness has always stopped me from getting into alternate tunings with any seriousness. I really like the style of guitar where you play scale runs over alternate strings, utilizing a lot of open strings in the process for that seamless "harp" sound where successive notes ring out, but I was always too lazy to tune strings to the radical kind of modal tunings that facilitate this (especially when the low E is dropped to a C). For this reason, I've developed the ability to do these kinds of runs in standard tuning, albeit with some pretty extreme stretches. I guess it's just easier for me to do the stretches than to make life easier for my fingers by retuning 

My second acoustic is also currently set to DADGAD as I am trying out a few songs in that tuning.
 
I have also experimented with Nashville tuning where you replace the wound strings with lighter gauge strings tuned an octave higher. This is really to play alongside a normally tuned guitar and gives a pseudo 12 string sound when both guitars play simultaneously which sounds great live or for recording.
 
A band I played in used to do The Rain Song and I used a Variax with a preset I made for the tuning -  D G C G C D.
Love the song and tuning but I don't know of any other song that uses it!
 

Thanks guys for these comments. I am getting a new acoustic next week and plan to standard tune it and try some alt tunings with my current one. So this gets me going.  thanks !!
 

ZincTrumpet
I have also experimented with Nashville tuning where you replace the wound strings with lighter gauge strings tuned an octave higher. This is really to play alongside a normally tuned guitar and gives a pseudo 12 string sound when both guitars play simultaneously which sounds great live or for recording.
 

That sounds like it may be the same kind of technique Davey Graham used for his piece "The Bloody Fields Of Flanders." It's a beautiful sound: 
 

 

My 6 string acoustic is regular tuning, and my 12 string acoustic is drop D and I use a capo to change keys if needed.

sharke

ZincTrumpet
I have also experimented with Nashville tuning where you replace the wound strings with lighter gauge strings tuned an octave higher. This is really to play alongside a normally tuned guitar and gives a pseudo 12 string sound when both guitars play simultaneously which sounds great live or for recording.
 

That sounds like it may be the same kind of technique Davey Graham used for his piece "The Bloody Fields Of Flanders." It's a beautiful sound: 
 

 

Yes that certainly sounds like Nashville tuning. Beautiful sound and Davey Graham's a great player.
P.S Your YT video wouldn't play from the UK but I VPN'd in from a USA server and it works fine.

 
I currently have guitars in standard, standard dropped a tone to D, open D and E (two guitars with very different tone and feel I use for slide), DADGAD and a C6/AM lap steel tuning. Plus a 3-string electric cigar-box in open F.
 
If forced to use two tunings it would be standard or standard dropped to D, and open D set up for slide playing.