I believe you first have to change a setting in umbracosettings.config:
<useDomainPrefixes>true</useDomainPrefixes>
Then I think you also have to right click on each top node and click 'Manage Hostnames', then add the appropriate host name for that top node.
It already sounds like you have IIS configured correctly, so you should be good to go on that front.
It's been a while since I've worked with Umbraco, but I think I'm mostly right ;-)
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