Providing you are using 2 different skins, you can set the default skin for each blog.

BackOffice->Blogs->YourBlog->Default display options->Default skin

I've done that and it still shows up the same way as my main template:

http://www.adamsweb.us/blog

Your skin choice has been remembered by cookies.

If you go into admin > blogs > blogname you'll see a box that says "allow skin switching" untick it and press save (on both blogs).

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