the act of baking desserts (cookies, cake, brownies) while getting stoned.
Not only is the subject of the action getting baked, but they are also baking goodies to snack on (which all stoned people love). Thus, the subject is double baking. This act usually involves preheation.
To listen to one's stereo while simultaneously lying on the couch watching television with the sound turned down.
This is based on the fact that those who simply lie on the couch watching TV are couch potatoes, therefore they must be "baking." It's even worse if they're doing two worthless things simultaneously: hence, they are "double baking."
It follows that they would be double baked couch potatoes for those who care.
"Are you just gonna lie there double baking, or are you goin' with me to Joey's?"