I'm using sandwich style beams for my deck. I'm using 4 - 4x4 posts
that are 6'4" apart and need a 20' beam. With a sandwich beam, what is
the best way to splice the beam at the interior post? Should I just
use 4 - 1/2" carriage bolts and put 2 into each beam segment into the
post? Hopefully my lousy diagram explains what I'm trying to do.

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Not sure exactly what you are saying. Are you thinking of bolting your
beam onto a post, mounted on the side?

I would suggest putting the beam directly on top of the posts. You can
purchase post attachments which will hold the beam in place.

I just built an 18' wide deck. My beam was 3 2x12's together, seated
directly on top of 6x6 posts. I trust the beam on posts much better
then the beam bolted to the post. However, I have seen the side mount
thing done.