Staying 40 days in Japan can I buy two 21 day passes?

Hi,
I will be visiting Japan for a total of 40 days in late September through early November. I will fly in and out of Narita Airport. I was interested in purchasing the JRPass for 21 days, but do not understand if I can purchase another 21 day pass to cover the second half of my travels. After the initial 21 days, can I buy another 21 day JRPass and if so can I do this while in Japan. Or am I ineligible to purchase such a pass after using the initial 21 day JRPass and thus I need to purchase JR line tickets individually once the 21 day pass expires. I will travel from Tokyo to Kyoto, Osaka, and eventually to Beppu and then back through the same areas again.
Thanks for any info!
LKnoerr