Why Septimus is considered to be Clarissa's double?

Woolf has stated unequivocally that Septimus was intended to be a double of Mrs. Dolloway. The two seem to be substitutes for each other. Woolf considered that he might live while Clarissa might die. Consequently many critics understand Septimus's death a swap for Clarissa's life. I know, this sounds strange but his death would allow Clarissa to accept the constraints of her married life.