Simultaneous rpc calls

What happens when both sides of a connection send a simultaneous rpc call, e.g.:

Parent:                Child:
NPP_DestroyStream      NPN_DestroyStream

Part 1:
How are the messages interleaved? Does this deadlock, or does the native
stack wind through both messages?

Parent stack:          Child stack:
0) NPN_DestroyStream   0) NPP_DestroyStream
1) IPC Code            1) IPC Code
2) NPP_DestroyStream   2) NPP_DestroyStream
3) Pluginhost code     3) Plugin code

Assuming it doesn't deadlock (since that would be bad!), I'm going to need
some way of gracefully dealing with this race: neither side has committed a
programming error.

Part 2:

Both of these calls map to the ~NPBrowserStream destructor. Will this
invalidate the actor handle as the message is sent? Since I don't think the
client code can prevent the race condition, the IPC/IPDL code will at least
need to gracefully handle the destructor being called simultaneously from
both sides.

--BDS