DNS is only concerned with host names not folder names. So if you are using multiple sites based on folders then you have the first site at the root using only the hostname:
and the second site in a folder like:
There is no additional DNS needed for the second site because its at the same hostname, hostname is resolved to an ip address by DNS, it does not care about folders.
If you use multiple sites with different hostnames you need a fixed ip address and you need DNS records to resolve each hostname to the ip address but with folder based sites at a single hostname you do not need this.
Hope it helps,