Translation:We had not seen each other for twenty years.
The verbs should be in agreement as far as time is concerned, as in:
"Nós não nos víamos HAVIA vinte anos." if it is passado imperfeito or "Nós não nos vimos há vinte anos." for the passado pretérito.
Pretérito imperfeito translates to: We haven't seen each other for twenty years. (accepted by DL).