The following scenario explains the above points.
One Central Manager (Central Manager 1) is standalone (not part of an MDR pair), and you want to add another Central Manager 2 to the standalone Central Manager 1 to form an MDR pair.
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If the communication between Managers and Central Manager 1 (that is, B and C in the figure) is over IPv4 network, the communication between Central Manager 1 and Central Manager 2 ( that is, A ) should also be configured for IPv4 network.
Similarly if the communication between Managers and Central Manager 1 (that is, B and C in the figure) is over IPv6 network, the communication between Central Manager 1 and Central Manager 2 ( that is, A) should also be configured for IPv6 network.
If the communication between Managers and Central Manager1 is over both IPv4 and IPv6 networks ( that is, B is in IPv4 and C is in IPv6), the communication between Central Managers 1 and 2 ( that is, A) can be configured to support either IPv4 or IPv6 networks.
Note
If you want to setup a MDR for the Managers, connect Primary Manager of MDR to the Central Manager. On the next MDR configuration synchronization, the Central Manager gets the details of the Secondary Manager and the Central Manager lists both the Primary Manager and the Secondary Manager.