菜鸟求问! 法铁的2 for 1 具体是怎样的？

那官网说
TGV duo offer conditions:
Single on TGV 2nd class
Travel with 2 persons
Depart from Paris & selected French stations
Offer valid from 8 October to 2 November, up to 3 days prior to departure
For duo travel from 18 October to 5 November

一定要跟两个人一起买么？
还有那日期是怎样算，是3天之后才能坐免费的返程吗
还有那个duo是啥意思...
菜鸟一个 求耐心的高手给我解答解答 跪谢！