What if someone took seaweed flour and put it around sushi?
You wouldn't make bread out of solely seaweed flour, anymore than you would make rye bread with only rye flour. You make bread with seaweed flour, or let's say rye flour as part of the mix.
I don't know what trying to make bread out of only seaweed flour would be like, but if it's anything near as bad as what would happened if you tried to make bread out of all rye flour, it's not something you'd want to attempt.
So, that said, if you took seaweed flour as part of your bread recipe, and made a bread out of it, and put it around sushi, then yes you would have a sandwich.
I've never tried to make bread containing seaweed, but I have made loaves of rye bread. I'm sure the way you wouldn't make a bread out of only seaweed.
Take the american biscuit. We can all agree it's bread. Yet it contains no yeast.
It does contain baking powder, which contains an acid element, and causes bubbles in the biscuit dough. The most basic biscuit contains flour, baking power, a liquid and shortening or butter. Some kind of fat in other words.
The most basic bread is made from flour, water, salt and yeast.
The most basic tortilla is made from flour, water, baking powder and lard. Again, a fat.
Hey! Wait a minute! That recipe for tortillas has the same ingredients as a biscuit! What the....?
But wait. Biscuit are a bread, made with flour, a rising agent, liquid, salt and a fat....and can be made in a skillet or in an oven.
But, tortillas, made with flour, a rising agent, liquid, salt and a fat, and is made in a skillet or some other flat surface...somehow isn't a bread?
If someone is hung up on the "2 pieces of bread" then consider than many times tacos are made with double tortillas.
Or, you can put the filling on one tortilla, put another tortilla on the top, and call it a quesadilla.
2 pieces of bread with filling in between.