The Observer has talked about concubinage before (as have other writers), so it's already been established that anyone who could have that sort of relationship with the Champion can have it even if you're already married to Kiyoko. Not to mention that literally none of the marriages already in the game are exclusive and none are going to be, so it would be rather odd to put that kind of restriction on concubines within the floofsphere when it doesn't apply to marriage overall.For those suggesting that the Champion (especially if soulbound with Keros) should be able to take Miko and Rindo as concubines do keep in mind. The Champion is themselves the Consort in his or her marriage and is, at the end of the day, not the authority of the household. I'm not certain a husband-consort would be entitled to a concubine, let alone two. xD
Now, whether Miko, Mai or Rindo fall into the category of potential concubines is another question.