I believe this is due to WordPress already seeing an admin user in the global user table, so on install, it won’t show a spot for a username. I was actually just messing with this exact same setup, and came across the same issue.
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If these are two both brand new site setups, you could delete the data in your users and usermeta table, and start the process of installing your second WordPress site. It should then give you the option to fill in all of the values for an admin user. Once installed, the new user you created will be an admin of your SECOND site only.
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Once installed, you will need to add some records into your usermeta table for your FIRST site installed. You should see two records in your usermeta table with meta_keys of “{table_prefix_of_second_site}_capabilities” and {table_prefix_of_second_site}_user_level. You will need to make the same key value pair for your FIRST site with the key of {table_prefix_of_first_site}_capabilities and value of a:1:{s:13:”administrator”;b:1;}, then another row with a key of {table_prefix_of_second_site}_user_level with a value of 10.
You should then be able to login to both sites with the admin user.