I don't know about 'bisexual'. Didi's always been pretty relaxed about showing her body around other girls, and is liberated enough to let anyone squeeze her boobs if they ask nicely (think back to when Gary suspected her of being transexual, and Zii followed her into the shower).
In this case, I don't think she's focused on "Zii and Yuki" being the important part of the act, as much as she is on "pinch and squeal" being a new and interesting sensation.
Although, I think Giz missed a trick by not having Didi's little shoulder-Angel being hogtied by her little shoulder-Devil, like Zii's usually is. It'd be an apt joke for the situation and a neat throwback to an older joke.