Atuendo/atavío

Atuendo/atavío
 

Are they synonymns?

Yes:
http://buscon.rae.es/draeI/SrvltCons...MA=atav%C3%ADo

I'd say "atavío" is much less frequent.

And even then, atavío has always been used when describing native attire.

There's some subtle difference.

Atuendo : the combination of clothes you are wearing right now.

Atavío : is the same , but normally said when talking about working clothes , or even native attire , as chileno described above.

Hope it helps !!