While I prefer to have each installation in a different database, it is possible to have more than one Limesurvey installation using the same database, by setting a different table prefix, as tpartner explained.
Now regarding the differen web addresses, this depends very much on your server environment. It is possible, if the server allows it. You can have different folders in the root and direct different URLs to it, but how to do that we can't explain, because every server can be configured differently.