In theory you could, it really depends on your network architecture and bandwidth between the sites. HA & DRS rely on shared storage to work, the ESX hosts at both sites must see the same storage and be managed by the same VIrtualCenter server. So presumably if you had ESX1 host at Site 1 using shared storage at Site 1 and had ESX2 host at Site 2 using shared storage at Site 1 (this would have to be a pretty high-speed link for it to work) then you could do this. You probably would not want to do it though, a better solution is to look at some of the replication and DR products that are available that replicate the storage between sites. Take a look at the products from Double Take, Vizioncore and VMware’s new product Site Recovery Manager instead.