I've been doing a lot of experimenting with 12 string capos lately. I have a Shubb.. 1 for the 6 string and the other for the 12 string. As much as I love Shubb capos, the 12 string version still gives me an issue on the bass E octave string (1979 Guild F512). So, I bought a Kyser 12 string capo, which has enough horsepower to retune the guitar up a half step! ...way too much.. it threw the guitar out of tune. I took it back to Guitar Center.
Now, I just got a Paige 12 string capo, and I think it is going to work well... very little pressure needed with the extra little rubber rings on it to make up the difference between the octave and the wound string thickness. It doesn't throw the wound strings out of pitch. I must say though, that I will probably try grooving the rubber on the Shubb to make it work, since I love those capos the best.