What are the fundamental differences between the ～と一緒に and the ～とともに fragments?


I'm accustomed to saying `together with` using the `~ to issho ni` fragment,
but I've been noticing that some people I talk to phrase this using `~ totomo
ni` instead.

i.e.

> 彼女と一緒に日本へ来た。  
>  Kanojo to issho ni Nihon e kita.

and

> 彼女とともに日本へきた。  
>  Kanojo totomo ni Nihon e kita.

seem to be fundamentally equivalent.

Are there any nuances that the two have that dictates when and where they
should be used? Probably very slight meaning deviations? Or are they
essentially perfect equivalents of each other when it comes to saying
`together with` (and yeah, I'm aware that `~ totomo ni` has other meanings
aside from that)?




一緒に, as you say, is regularly used for saying doing something "together".
共に(ともに) is more explicit about who you are performing this action with,
thereby placing a stronger emphasis on the bond/camaraderie.

私と一緒に戦います!

Fight together with me!

私と共に戦います!

Fight hand-in-hand/side-by-side with me!

So I would say you'd pick which to use depending on whether you are trying to
place an emphasis on the action or the other person(s) involved.




According to page 91 of the 類義語使い分け辞典 [1], [quoted
here](http://nihon5ch.net/contents/bbs-study/old/mie-bbs.cgi?s=165), 一緒に
requires that the action take place in the same time and location. 共に does not
have this restriction, so the subjects may perform the action (let's say
日本へ行く) at the same time but via a different route, or via the same route but
at slightly different times. 「彼女と一緒に日本へ来た」 means that you and she came to
Japan at the same time _and_ were together the entire way, but 「彼女とともに日本へきた」
_could_ mean that you came to Japan at the same time, but took different
flights to get there.

To provide another example (extracted from Google), consider
「一緒に学ぼう！共にがんばろう！」. The first sentence shows that the action, 学ぶ, is being done
at the same time and in the same location. The second sentence uses 共に instead
of 一緒に, and so the best interpretation is probably that the action, がんばる, is
being done by everyone at the same time, but in their respective locations.

Sources:

  1. [類義語使い分け辞典](http://books.google.com/books?id=J5MxAQAAIAAJ) (Google Books)

