mirror and unite solid - faces change

 From:  Martin (MARTINK)
8157.1 
Hi there,

I'm new to MoI, worked some tutorials and try to be productive actually :)

I want to create a solid on which I can use Max Smirnov's SubD script. The solid should be symmetrical, so I thought I can design only one half, mirror and unite it.
So this would be the half solid:


I selected and mirrored:


Then I selected both solids and united them:


And there's my problem. The faces in the last picture are not symmetrical anymore. On the bottom an L-shaped face was generated, that would cause problems in further use. I tried this procedure according to this thread: http://moi3d.com/forum/index.php?webtag=MOI&msg=7383.4 As you can see it worked great there, so I think I'm just doing something wrong.

Kind Regards,
Martin