Would it be possible/wise to execute 2 parallel restores from a single backup?
I am pretty sure that I have done this, though I didn't think twice about it at the time. 

You definitely can restore a bak file using a UNC path. Mapping drive letters is to be avoided because the mapping must be done in the context of the SQL service account. Most people map the drive under their own account and are frustrated when the restore command doesn't work as they expect.

Naturally, there will be a little more load on the NIC on the server that holds the BAK file and there could be some contention if the database mdf and ldf files are on the same SAN, but these are probably minor problems, if you notice them at all.