code: public <P>JsonModelBuilder<T> add(JsonPropertyBuilderCreator<T> creator1,
  			JsonPropertyBuilderCreator<T> creator2) {
  		addSub(creator1.<P> get());
  		addSub(creator2.<P> get());
  		return this;
  	}
docs: Attaches property builder for the given types.
  @param creator1
  @param creator2
  @return this
  @author vvakame