James, if those "two Moodle servers" serve the same Moodle site, same $CFG->wwwroot, then what Howard described applies, with the additional point https://docs.moodle.org/en/Session_handling. Either the two servers share the session data through the common database (more resources) or can store locally (more efficient) if your load balance is "session aware".
This topic has been discussed many times in the Hardware and performance forum. But there is no Moodle-preferred method, simply because the architecture depends on the server environment you use.
Installing and upgrading help
Installing two Moodle servers and one database server
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