using a layer's threshold to create layer mask

I have 2 exposures of a photo.  One with building exposed
correctly but the sky blown out.  The other with the sky exposed
but building too dark.  Want to use the second sky on the first
building.

Have both photos in different layers and want to create a layer
mask to blend them.  I can do it manually, but thought that by
using colours->threshold would be a good way of selecting the
areas I want masked.

Any tips?

I duplicated the good sky shot and have it black and white by
using threshold like what I want the layer mask to be, but can't
get it into the layer mask other than just to use colour select
and bucket fill.

-- 
Troy Piggins | http://piggo.com/~troy
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