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There are a couple of advantages of the sandwiched 2x4's.
1. By sandwiching you can achieve a structural 24' length, sandwiching is a kind of laminating by having the joints offset to each other the loads are transfered much more effectively to the next piece.
2. by sandwiching the 2x4' you will in effect end up with a 4x6.
If you do sandwich then naturally the 2x4's must be installed side up.
How do you intend to join the 2 4x4's end to end, unless you use steel plates on either side and bolt them together, even then this method won't stop the 4x4's sagging at the joint over time.
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