Hey there,
Lina was initially intended to be a less important companion, and a "compensation" for losing out on Ava. That character became a little bit more than that considering it now has three possible evolution paths. It's technically possible to allow the players to have both, but we will have to adjust the game accordingly and rewrite some content to support it. It's unlikely we'll do it now, but could be something we'll look into after the campaign is finished.