An interventional medical system comprising: an implantable medical device comprising: a ventricular portion including an hermetically sealed housing extending from a proximal end thereof to a distal end thereof, a ventricular electrode mounted in proximity the housing distal end, and a fixation mechanism mounted in proximity to the housing distal end; an atrial portion including a core extending from a first end thereof to a second end thereof, along a longitudinal axis of the atrial portion, an atrial electrode mounted in proximity to the second end of the core, and a fixation mechanism mounted in proximity to the second end of the core; and a leadlet extending from the proximal end of the ventricular portion housing to the first end of the atrial portion core, the leadlet including a conductor and an insulative tubular member in which the conductor extends, the conductor being coupled to the atrial electrode of the atrial portion; and a delivery tool comprising: a tubular sidewall that defines a lumen and has a slot formed therein, the slot extending proximally from an open end thereof, the open end of the slot being coincident with a distal opening of the lumen; and a tether extending within the tubular sidewall; wherein the device is loaded in the delivery tool such that the second end of the atrial portion core faces the proximal end of the ventricular portion housing, such that the atrial portion is contained within the lumen with a segment of the flexible leadlet extending alongside the atrial portion, and with another segment of the leadlet being folded over on itself proximal to the atrial portion, and such that the tether is engaged with the other segment of the leadlet at a fold therein; and the slot is configured to allow passage of the flexible leadlet therethrough when the atrial portion is positioned in proximity to the distal opening of the lumen for deployment from the tool. an implantable medical device comprising: a ventricular portion including an hermetically sealed housing extending from a proximal end thereof to a distal end thereof, a ventricular electrode mounted in proximity the housing distal end, and a fixation mechanism mounted in proximity to the housing distal end; an atrial portion including a core extending from a first end thereof to a second end thereof, along a longitudinal axis of the atrial portion, an atrial electrode mounted in proximity to the second end of the core, and a fixation mechanism mounted in proximity to the second end of the core; and a leadlet extending from the proximal end of the ventricular portion housing to the first end of the atrial portion core, the leadlet including a conductor and an insulative tubular member in which the conductor extends, the conductor being coupled to the atrial electrode of the atrial portion; and a ventricular portion including an hermetically sealed housing extending from a proximal end thereof to a distal end thereof, a ventricular electrode mounted in proximity the housing distal end, and a fixation mechanism mounted in proximity to the housing distal end; an atrial portion including a core extending from a first end thereof to a second end thereof, along a longitudinal axis of the atrial portion, an atrial electrode mounted in proximity to the second end of the core, and a fixation mechanism mounted in proximity to the second end of the core; and a leadlet extending from the proximal end of the ventricular portion housing to the first end of the atrial portion core, the leadlet including a conductor and an insulative tubular member in which the conductor extends, the conductor being coupled to the atrial electrode of the atrial portion; and a delivery tool comprising: a tubular sidewall that defines a lumen and has a slot formed therein, the slot extending proximally from an open end thereof, the open end of the slot being coincident with a distal opening of the lumen; and a tether extending within the tubular sidewall; a tubular sidewall that defines a lumen and has a slot formed therein, the slot extending proximally from an open end thereof, the open end of the slot being coincident with a distal opening of the lumen; and a tether extending within the tubular sidewall; wherein the device is loaded in the delivery tool such that the second end of the atrial portion core faces the proximal end of the ventricular portion housing, such that the atrial portion is contained within the lumen with a segment of the flexible leadlet extending alongside the atrial portion, and with another segment of the leadlet being folded over on itself proximal to the atrial portion, and such that the tether is engaged with the other segment of the leadlet at a fold therein; and the slot is configured to allow passage of the flexible leadlet therethrough when the atrial portion is positioned in proximity to the distal opening of the lumen for deployment from the tool.