Geneshaft

both are mostly set in space, a lot of drama and great music. Geneshaft is more action oriented while Mugen no Ryvius is a bit darker exploring the nature of human interactions when there is not any clear authority. Also both have a mecha robot in a space ship built from alien technology 

both are mostly set in space, a lot of drama and great music both share the theme of a society that has been devoid to a certain extent of emotions but one or more of the characters still has a normal range of emotions. Geneshaft is more action oriented while Seikai has more politics and relationships 

Both have some culture shock when people who have been living very different lives are placed together.  

they are both mecha and follow the same lead, the lead characters both have trouble and are trying to figure out where they belong 

These two have almost identical plots and similar sci-fi apocolypse themes. Geneshaft also has a massive amount of girls, much like Divergence Eve.