[TYPO3] Can I get mydomain.com/subsite to load a typo3 root page

[Another Vision]

Hi List
 
Sorry if this is a really obvious one, but I've been tinkering with domain
records and realURL for about a day and a little wisdom would be much
appreciated.
 
What I'd like is www.mydomain.com/subsite to load the root page of a
secondary typo3 site while www.mydomain.com loads the main site.
 
Ideally links followed within subsite would then be realURL's like the main
site so www.mydomain.com/subsite/xxx/page.html What's foxing me is all the
documentation I'm reading relates to routing alternative domains. I've tried
a Typo3 'domain' record of mydomain.com/subsite , but I suspect the whole
thing is solvable in realURL if I only knew what I was doing.
 
real URL has lots of options which seem to make this possible, but I can't
work out where I'm going wrong or which is the best route to take.
 
Can anyone give me some pointers?
 
Thanks alot
Mike