ou can do them both in parallel in the sense of simultaneous operations on the same datastore, as well as in parallel across datastores.
First - simultaneous operations on the same datastore.... Regarding If you look at this guide page 246.
"VMware Infrastructure 3 supports a maximum of four simultaneous VMotion or Storage VMotion accesses to a single datastore. A migration with VMotion involves two simultaneous accesses to the datastore, by the source and destination hosts. A migration with Storage VMotion involves one access to the source datastore and one access to the destination datastore. Therefore, if no other migrations are occurring, up to four concurrent Storage VMotion migrations involving the datastore can occur simultaneously."
BUT - in my experience, you don't net a lot from this. Normally with a single filecopy, there's some benefit of simultaneous multi-threaded copy processes, but remember, it's likely that this datastore already has a bunch of IO that it's doing. Can it help? Sometimes - but personally, I'm all for the next thing....
Second - parallel across datastores.... Absolutely leverage this. If you do it - you can get N jobs at once, where N is the number of datastores you have on the cluster, but cannot exceed 32 (because you can only have one SVmotion per ESX host, and a max of 32 per cluster with VI 3.5)
