Hi, I have just posted the same here, so just for the record. Japan doesn’t have a 2G network (at least regarding Softbank and NTT Docomo, but with a foreign SIM card these are the only ones you can use), and thus it is not possible to use 2 SIMs at the same time, as only one SIM can be assigned 3G connectivity.
More info on Japanese mobile coverage and SIM cards, here: http://prepaid-data-sim-card.wikia.com/wiki/Japan