Most of the gambling sites have mirror sites but as far as I know these are for the purpose of load balancing. For example, if there were no mirror sites, all the load would be on the same site but since the load can be divided by mirroring, it helps the gambling site to operate more efficiently.
that's not how load balancing works. load balancing is done on the server side, so you don't need mirror sites with different domain names for that... mirror sites are mainly for accessibility in case the main domain is taken down, not working properly, or to circumvent ISP/geo blocks.