If your blogger site redirects to your own domain then only your domain will have a sitemap since the content is stored on your domain not the blogger. Google will eventually deindex the blogger URL's and your domain will become the new master, therefor again no need for a second sitemap.
www.url.mydomainname.com to webmaster tools and inform Google that your site has moved to your custom domain. You can tell Google that your site has moved by doing the following:
- On the Webmaster Tools Home page, click the site you want.
- Click the gear icon , and then click Change of Address.
- After completing steps 1-3, click Select a verified site to select the new site.
Then simply generate a new site map and submit it under the custom domain account and not that of Blogger. Attempting to submit 2 sitemaps would be a waste of time as the blogger site will soon be deindexed and thew new master will become current.