If you're talking about managing MULTIPLE websites, then you will need to (obviously) create two separate sitemap.xml files and upload to the respective websites. I would recommend using this service: http://www.xml-sitemaps.com/. At the same time - YES you can add upto any amount of URLS in one sitemap file as long as they belong to one WEBSITE/DOMAIN. To submit the sitemap of your websites. Go to "Google Webmasters" add both your websites and submit sitemaps respectively. Thanks
First ask your questions properly, so that people will answer it? now reply what are asking about. Have different sitemaps for different(multiple) websites. Are you asking about multilingual sites.
You should *not* include session IDs in your sitemap URLs! (e.g. A1 Sitemap Generator can cut these off, so can some other tools. If your current tool does not support your website session IDs, contact the author of it for help)