Well yes, most offices only issue the 60 days visiting Thai dependants. Never seen an Immigration office issuing both the 30 and 60 days extention for the same visa/exemption/entry.
It doesn't matter what it's issued for, you can apply for and most usually get a 60 days extention based on having Thai dependant relatives if you have Thai dependant relatives. You don't have to be married, maybe have a half Thai child.