If you already have an IIS site pointing to your DNN install folder and you are using Host Headers in IIS then you will need to either use a different IP address for the administrator site, or use a different Host Header on the new IIS website.

If you use a different Host Header, then you will need a PortalAlias setup to handle that in DNN.  Call it something like "admin.smallbizserver.net"

Just as a refresher, or for those who may not know :
Host Headers in IIS are used to determine the IIS website being requested when more then one IIS website shares the same IP address. 
The PortalAlias is used by DNN to determine which Portal in the system is being requested, which happens after IIS has determined which IIS website is handling the request.

Ok, lets step "above" IIS into DNS. 

Yes, you can have two (or more) A records with the same host name that point to different IP addresses.  This is how some people setup Round-Robin DNS which lets your DNS server take turns handing out IP addresses when requests come in.  If those IP addresses were actually on different machines then you could get a cheap load balancing that way.

But in this case we want them to go to different  IIS websites which can also be on the same machine.  If they are using the same hostname then DNS will not know which IP address you really want and it will just give you the next one that matches your domain lookup.  So you still need another hostname to make sure you get to your administrative portal. 

Then again, you could just skip DNS and put the IP address into the browser directly, in which case you'll need a PortalAlias to match the IP.

By the way, if there are a lot of updates being made to your site by content editors, etc.  This is a good idea regardless of if you are using Web Gardens or not.  The reason being that if you are doing administrative tasks you will generaly be tying up the thread your request is on for longer amounts of time.  So if you had a seperate set of threads for admin and normal browsing your normal browsing users will see better response times.

Of course that last point also becomes moot if you are doing Page Caching. :)