Trimmers are designed to support the total load without crushing the fibers in the wood beam where it rests on the trimmer or from crushing the trimmer.
The engineer notes in the letter that the new beam is to support the second floor (one floor). There are no dimensions given, but using 2x8 floor joists at 16” on center, They can span about 12’-9” maximum, depending on the species and grade.
So the floor load on the new beam is about:
50 psf (Live Load of 40 psf plus a Dead Load of 10 psf) x 12’-9” = 637.5 plf
So the load at each end is:
637.5 plf x 6’-6” / 2 = 2,072 lbs.
One 2x4 trimmer has an area of:
1.5 x 3.5 = 5.25 sq. Inches
Therefore the stress on the trimmer and the new beam at each end is:
2,072 lbs. / 5.25 sq. In. = 394 per sq. in.
Wood in compression parallel to grain (the trimmer) can support about 1,100 lbs. per square inch, which is significantly under stressed. However, wood in compression perpendicular to grain (the new beam) can only support about 500 lbs. per square inch, which is slightly greater than the actual 394 lbs. per square inch.
So, one trimmer is acceptable, provided: 1) the hose is not more than 25’ wide, 2) there is no roof load on the existing wall, 3) the LVL’s are Douglas Fir No. 1 or better.
The most important thing is to verify that the engineer accounted for all the loads on the new beam, INCLUDING any roof loads. I’d look in the attic and see if any ceiling joists or roof joists bear on this wall and ultimately the new beam. If so, I’d notify the engineer immediately. (As he says in his letter, he wants to know if any of the conditions are different than what he has shown.)
Btw, I’m surprised he did not stamp the drawings. It’s required in every state.
(I think you’ll find out he’s a civil engineer, not a structural engineer.)