It looks like the Nikko-Kix has triple (3 beds) rooms. If your twins can share a twin sized bed, that might work. You might want to book well in advance. It looks like those rooms are sold out for September and October. They seem to have few in November but good availability for December, so 3 months in advance or so - more around busy times.

https://asp.hotel-story.ne.jp/ver3d/...SELECTCOD2=001