An implantable system includes a first leadless pacemaker (LP1) implanted in or on a first chamber of a heart and a second leadless pacemaker (LP2) implanted in or on a second chamber of the heart. The LP1 uses at least two of its electrodes to transmit and receive implant-to-implant (i2i) messages to and from the LP2. During one or more periods of time, the LP1 times delivery of pacing pulse(s) to the first chamber of the heart based on timing of cardiac activity associated with the second chamber of the heart detected by the LP1 itself. During one or more further periods of time, the LP1 times delivery of pacing pulse(s) to the first chamber of the heart based on timing of cardiac activity associated with the second chamber of the heart as determined based on one or more i2i messages received by the LP1 from the LP2.
