I've got a bucket in Google Cloud Storage and I've got two static web sites inside.
I've got two subdomains:
Is it possible to use the 1st sub-bucket for the 1st subdomain, and the 2nd sub-bucket for the 2nd subdomain?
I've found this https://stackoverflow.com/a/24236186/2833774 But not sure if I correctly understand if GCS has limitation about this or not.
This can not be done on the way you describe, instead you will have to use one bucket for the domain and one bucket per subdomain.
First, you create a bucket whose name must be the same as the domain you point to. Then, you can validate it with a CNAME record.
After all that process, you can then create a second bucket with the subdomain name (because you verifed the domain name in the step before)