I think celebs go to each others concerts all the time
Namie Amuro went to Kuu's Kingdom Tour, and I believe Ayu has attended Kuu's tours before. It's nice to know they all get involved with each other and take inspiration of each other.
Kuu and Ayu always have a nice chat when they see each other, I remember they sat next to each other when they both went to see the 'Mr & Mrs Smith' premier in Japan.
It's nice to see.
And I am totally voting for the sexy Kouhaku outfit.