We don't have synonyms for evener yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are sevener, evene and avener.
(n) :
(dated) In vehicles, a swinging crossbar, to the ends of which other crossbars, or whiffletrees, are hung, to equalize the draught when two or three horses are used abreast.
(n) :
(dated) In vehicles, a swinging crossbar, to the ends of which other crossbars, or whiffletrees, are hung, to equalize the draught when two or three horses are used abreast.