Is there a way to add blend shape targets once you perform some other operation on the master obj?

I'm modeling my targets with instanced symmetry -which works fine (my master model is instanced symmetry too). This way I just have to model 1 side of the face...

After I've loaded all the targets into the blend, I want to combine the 2 halves and merge the vertices. This works fine too. However now I can't add more targets to the blend node - which is below (or up-stream from) the combine node. Is there a way around this? Does anybody model morph targets with symmetry or is it just me doing something wrong...?

Also, when creating Blend Shape, it has the option (under Advanced) for Deformation order (Before, After etc). I was thinking it might have to do with the above. But after creation, how to you access these options again? I don't see it anywhere in the attribute editor...