One MVNO that offers a rental service:
https://opr.inphonix.co.jp/j-mobiler/in/info.cfm?ref=19

I think it might be somewhat cheaper than the major carriers who have booths at the airport - but still quite steep IMHO. The per-minute charges are, however, competitive with other pre-pay phones.

If you have friends in Japan, I am almost sure that you would save money by asking them to buy two prepay phones for you to use during your time in Japan. (You need to be a Japanese resident to buy a pre-pay mobile phone).