I think that two restores of the SAME filespec at the same time aren't
allowed; but two restores of DIFFERENT filespec can run on the same client
at the same time.  I just started up two restore sessions on my current PC,
just to make sure it still works; both are running now.

Again it doesn't make sense much sense if both restores end up using the
same single tape.  And it is only useful up to the point where you exceed
the capacity of your network, or if you are trying to write to the same hard
disk.

But we have done restores of multiple AIX filesystems in parallel before,
very effectively, when they were spread over multiple tapes.