Bo,
It's not that simple, unfortunately. A thin patty gets overcooked and dried out very quickly. Stacking two just gives you a thicker dried-out and over-charred well-done sandwich. You've got to start with a thicker patty to retain juiciness and flavor.
It's just like the steaks - two of X's notoriously thin NY strips does not equal one thicker one that can be cooked to a perfect interior rare or medium-rare.