Is an endotoxin an A-B toxin?

The term "endotoxin" is a bit wonky. Originally, it was intended solely to mean a toxin(s) not secreted by the bacterium cell until after cell death. Since then, however, the term has come to refer to the cell wall components of Gram-negative bacteria, most specifically lipopolysaccharide (LPS).

In the case of the term "endotoxin" referring to LPS, then, no -- LPS is a sole toxin (an "A"). However, when the more general meaning of the term "endotoxin" is used, the toxin released may be an A-B toxin.