Is this supposd to be an exterior beam? If so flash the top and forget
about
the shims.
If it is not exposed to weather forget about the shims.
If the 2x10s are placed side by side (oriented vertically) then it
hardly
matters at all whether they are attached to each other. If they are
properly
blocked to prevent rotation and buckling the there is no force that is

Correct but he's referring to a BEAM which, in most parts of the
country, is considered to be a horizontal cross member which supports a
load.

I am too. By vertically I mean that the beam is 9 1/4" deep with the 2x side
by side instead of stacked (which would be a bad idea).

He further indicates this by referring to it as a 12' HEADER.
You're speaking of a post or pilingg

Actually I DID write about a beam. With a post, bonding the two (or more)
together is MORE necessary to prevent buckling.

I constructed a header for a 16' overhead door opening on the garage I
built. The sidewall containing the open was load-bearing for a truss
roof. I laminated a ¼" ~17'x9" steel flitch plate between the 2x10's
and bolted the whole thing together with carriage bolts as he proposes
(staggered). The garage is now 20 years old and there is absolutely NO
sag in that header - none. It'll take a fire to make that puppy sag.
Overbuilt? I dunno. That's what the plans called for in the header.

A 17' load bearing span is definitely too much for a couple of 2x10's. In
your application, the steel is doing most of the work and the wood is there
to keep it from buckling (also to nail to I presume). I'd hardly consider
that to be overbuilt.

IIRC correctly the ¼" steel flitch plate w/bolts may have been an
alternative to either ½" or 3/4" ply nailed.

Without knowing the loads, it is hard to say, but it is quite unlikely that
you could have substituted a 9" wide piece of 1/2" plywood for the steel.
Making the beam deeper (perhaps using the plywood as a web) would certainly
help. Beam deflection decreases with the cube of the beam depth.

BX1's best bet is to check with Building/Zoning or Community Development
in his town and see what they say.

Good advice. This is really a simple case and they should be able to give
him an answer based on codes.

To overbuild is never a crimeg

Actually I consider it equivalent to theft if I am the one paying for it. If
not, then it is simply good practice.

-j