the problem with your code is that your code says the second rest is in
\oneVoice (this means there are no other simultaneous voices around).
But there is the c' at the same time in \voiceTwo and that confuses
LilyPond. If you mark the voices correctly, you will get this:
\version "2.19.32"
{
\new Staff <<
\time 3/4
\new Voice { \oneVoice r4 \voiceOne r g' }
\new Voice { \oneVoice s4 \voiceTwo c'2 }
>>
}
Now the rest is on the top line emphasizing that it belongs to the upper
of simultaneous voice. Which is ok, but perhaps exaggerated. If you want
to reproduce your original measure exactly,
\override Rest.staff-position = 0 helps as Ben said.