Is it possible to have same web application with two site collections having content databases in two different SQL servers?
If so, can farm back and restore method from CA be used for restoration?
you can have multiple database across multiple SQL server for the same web applications. I have same kind of requirement and working without any issue.
Yes farm backup and restore method will work as well, because when you perform the restore it will ask you where to restore. at that time you can tell the sql instance name.