Paul is correct. Strings 11 & 12 were two octaves apart. He also tuned strings 5 & 6 in unison using identical strings. The other major difference is Lead Belly tuned his Stella further down then most, frequently it was tuned to C rather than E and sometimes B or even Bb. As Gene E. said, concert pitch is for them that wants to play with other people. Otherwise, you can use any pitch you wish. There are stories that Lead Belly chose pitch based on the condition of his strings. I suspect that is apocryphal. Also, as Gene mentioned. If you tune as low as Lead Belly you will need heavier gauge strings. One person I know uses 0.64 gauge on his 11th string.

I have a 1968 Guild 12-string. I keep it tuned to concert pitch. Guild built the guitar with that in mind. 12-strings built earlier often were tuned to D to ease tension on the neck. I would suggest people contact manufacturers if they are concerned. If you don't know, then safety might call for tuning to D.

I have been very satisfied with every capo designed for 12-strings except the old double elastic ones from the '60's.