So your concerns/questions are not clear at all.

Try again explaining what you had wanted, what he wants, what your concerns are, what does the structure in question need to do. We aren't standing at your desk looking at the pictures/drawings with you. We need you to paint a full picture.

So the contractor doesn't want to build a wall that will stand up? That's his problem. Can you put a double #5 in an 8" block - possibly. Is it a good idea? No, not really. Mortar fins would have to be non-existent, splicing will be very difficult, grout will have to be very fine. There are a lot of ways to screw it up and the contractor already sounds like somebody who doesn't really care what the engineer has to say. Give him something difficult to build that will be hidden until structure falls down? Yea...he'll build it that way...

And why, exactly, did you put a masonry retaining wall question in the wood design forum?

Very unclear question. How about some basic additional information such as height of retaining wall, footing size, etc..., better yet, how about you sketch a cross section. Based upon the information provided to date, I am really curious about the footing that this 8" block retaining wall attaches to, which I imagine will lead to other questions.

lol most confusing question ever