Google already provides you a tutorial on how you can add a domain to your blogger blog. But it does not specifically show the steps for setting up two domains - 1. Main Domain 2. Subdomain for your two blogger blogs i.e. you will have two blogs one on main domain and the other on subdomain for example http://www.allbloggertricks.com and http://demo.allbloggertricks.com.
Below are 3 easy steps with images on how you can add a subdomain to your blogs existing domain without any difficulty.
Step 1 : Add a A Name Record
Go to your domains registrar website and click on Change DNS/DNS Manager option. There you would find different CNAME and A records you have made previously for your main blog. Add another A name record with the the desired subdomain as your host. For example if you want subdomain http://demo.allbloggertricks.com/ for your blog then enter demo in the host name. In the IP/Point to section add this IP - 126.96.36.199 After adding save the record.
Step 2 : Setup the domain in Blogger
Now go to Blog Title → Settings → Basic → Blog Address → Add a Custom Domain → Switch to Advance Settings and add your full subdomain in our case it would be demo.allbloggertricks.com. Make sure the box saying "Redirect allbloggertricks.com to demo.allbloggertricks.com" is unchecked else when someone goes to allbloggertricks.com (with no www ) they will be redirected to http://demo.allbloggertricks.com/
Step 3 : Settings in the main Blog (Required in some cases)
Even after unchecking the box above, if the domain http://allbloggertricks.com (without www) redirects you to http://demo.allbloggertricks.com Go to Main Blog Title → Settings → Basic → Blog Address → Edit and check the box which states "Redirect allbloggertricks.com to www.allbloggertricks.com" Thats it, the whole setup is done. Sometimes if takes 1-2 hours for making the changes by the domain registrar but most of the time change is withing 5 minutes. Your custom subdomain is ready.