Can 共に be used as a substitute for 一緒?


I understand 一緒 cannot be substituted with 共に given how flexible 共に is and how
people-specific 一緒 is.

But can the reverse be true since 共に can also be used with people?

例：友と共にパーティに行った。




「共{とも}に」 and 「一緒{いっしょ}に」 certainly share the meaning of " ** _with ~~_** ".

The important difference, however, is in the formality levels of the two
phrases. While 「共に」 sounds formal and slightly stiff, 「一緒に」 sounds informal
and conversational.

In our informal daily conversations with family members, coworkers, friends,
etc. the vast majority of us would almost exclusively use 「一緒に」. 「共に」 simply
sounds too serious for daily use.

What this means is that if you used the two interchangeably, your "new"
sentence is bound to sound awkward because that is what inevitably happens if
you change just one word out of a whole sentence to another with a different
level of formality. Your new sentence will just lack the good balance with one
of the words sticking out as a poor word choice.

Your sentence:

> 「友{とも}と共にパーティに行{い}った。」

sounds borderline okay because 「友」is already stiff compared with 「友だち」. Stiff-
on-stiff is alright.

If you asked me, however, if any native speaker would utter that sentence
verbatim in real life, I would honestly reply that the chances would be very
close zilch. It is **_far_** more natural to say:

> 「友だちと一緒にパーティーに行った。」

