Eran las cartas de amor entre mi abuelo y mi abuela durante la guerra, cada uno en un bando.

Hola Peter

Here "cada uno en un bando" (each one in a [different] side/bloc] is referring to her grandfather and her grandmother. The text is not talking about any specific war but this is referring to the Spanish civil war in the 30s. There were two main sides:  el bando republicano and el bando nacional. 

We use the default masculine "uno" when we are talking about two people, one masculine and one feminine. 

I hope this clarified it. 

Saludos

Inma