Yes it is possible:
Point a subdomain to a different Squarespace site
When you point a subdomain from one Squarespace site to another, Squarespace handles it like a third-party domain. This means you’ll use DNS records to connect it to the second site.
Start with the site you’re pointing from. Open Advanced DNS settings for the root domain that you’ll add the subdomain to.
From the Add Presets menu, select Subdomain. Enter the new subdomain name, such as blog or store, then click Add.
Go to the second site where the subdomain will point. In the Domains panel, click Use a Domain I Own.
Enter the full subdomain. For example, blog.yourdomain.com.
Click Connect From Provider.
Select Other as the domain provider, then click Connect Domain.
From the DNS Settings panel that appears, copy the unique code from the Host column.
Return to the first site’s Advanced Settings. Scroll down to Custom Records to create a new record.
From the Record drop-down menu, select CNAME.
In the Host field, paste the unique code.
In the Data field, enter verify.squarespace.com
Click Add, then click Save at the top of the page to add the record.
Return to the second site. In the Domains panel, disable the WWW prefix for the subdomain.
Test the connection by entering the subdomain in an incognito window.
