The second system IPL

Figure 1 shows the multisystem environment after SYSB is initialized.

Figure 1. Two Systems in the Multisystem Sysplex

When SYSB is initialized and joins the sysplex and the global resource serialization complex:

XCF coupling services are available on SYSA and SYSB, including signaling services between SYSA and SYSB.
The same primary sysplex couple data set is shared by SYSA and SYSB (not shown).
MVS™ component exploiters (such as global resource serialization) use signaling services between SYSA and SYSB but must use non-XCF connections to communicate with the other systems.
Multisystem applications can exploit XCF coupling services and signaling services on SYSA and SYSB.
SYSA and SYSB must share a Sysplex Timer if they are running on separate CPCs (not shown).

Consider this mixed complex a temporary configuration during migration until the sysplex matches the complex.