I have found that with a pair of them interconnected via null-modem cable, it is better to have the tty00 devices disabled in /etc/ttys, so that they do not interfere with one another while in multiuser mode (normal operation).
During maintenance, such as with bsd.rd for upgrades or while in single user mode, the /dev/tty00 serial port console is used without /dev/ttys. One merely need ssh into the operational platform and then connect to the system under maintenance with cu(1).
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