Incorrect information here.
Allow me to correct it.
Importing 2 forums into 1 XenForo is easily done. You import the first (usually the biggest (though it doesnt matter in reality)) forum first, and save the import log as import_log_1. Set up the redirection script on your first forum pointing to import_log_1.
Next, import forum 2 and set it to automatically merge users with the same email address. Save the import log as import_log_2. Set up the redirection scripts on the second forum pointing to import_log_2.
Job done