I found this  http://codex.wordpress.org/Installing_Multiple_Blogs

And this

http://wordpress.org/support/topic/two-wordpress-installations-one-domain-name-and-host

Hi Jean

That is a standard warning.  If I remember rightly you are trying to overwrite something in the root which is where I usually put them anyway.

Are you using an add-on or sub-domain - I use sub-domains and you can install a new wordpress in each one (I actually have three wordpress accounts on mine, one of the main domain and one each on two sub-domains)

As long as you are sure you are loading it onto the subdomain
this was a tutorial I created which may or may not help
http://www.microstockgroup.com/symbiostock-hosting/installing-wordpress-%28via-justhost%29/

Thank you Tinny; in fact it is an addon as I want to use a completely different name.
I supose subdomains are having names related to the main domain?
Jean

subdomains are a separate issue

each wp-sym site needs it's own folder - I put mine in /pix and /symbiotest and then reference them by http://cascoly-images.com/pix  etc

a subdomain is a naming convention like travel.cascoly-images.com  which would then be set to point to http://cascoly-images.com/pix or whatever.  by default a new subdomain points to the root

yet another option to buy additional domains from the same host -- eg cascolytravel.com but again  these all point to the root  by default. 

finally you could get another hosting package entirely -- I currently have 2 sites with 1and1 (one MS and the other Linux), and one with justhost