Our family is build both by birth and adoption. Foreign adoption is very hard and very expensive. I do not think Japan allows many adoptions. Also incase of dissasters like this it is often the children that do not survive as was shown with tsunamies in other places. That said the need is very high especially if you are able and open to age, gender, race of the child being placed.
As for adoption two children at the same time, it is the only thing my children that were adopted would recommend. They would not want to be the only adopted child in the family and in your case it would possibily be the only girl. My children did not care what race the other child would be as long as it is also an adopted child.
Race is overal a none issue to them but we live in a diversed area. This does not mean it is a none issues to outsiders and as your child grows older they need to be prepaered for that. I do notice that as they get older they tend to have more friends that are similar to them in one way or another race, cultural or adopted but you might also find that your biological child is more open to race and adoptive friends. Adoption is very powerful just like birth. If you and your husband are open to adoption, please look into it.