Travel between Beppu and Hakata

Hi, you mentioned that for travel between Beppu and Hakata, a set of two tickets (2枚きっぷ, nimaikippu) for use by one or two persons in either direction within a one month period is available for only 6,600 yen. Is that only available for purchase at the train station or can I buy it online? Thanks!

2 ticket/nimai kippu =6600 yen
Like 18 kippu,you can use it by yourself in either direction or use it with your 1 friend
So Beppu to hakata=3300 yen,(1 way cost)

But do you know,how i explore beppu,hakata as well as other north kyushu region?
By sunq bus pass 3 day=9000 yen,unlimited

The 2枚きっぷ cannot be purchased online.
Here is the offical page describing the tickets: https://www.jrkyushu-kippu.jp/fare/ticket/17

インターネットでの発売は行っておりません

Thanks everyone. Much appreciated!