Crash running two instances of the game simultaneously on Windows

[vts]

When spawning two instances of the game on the same machine, hosting a game on one with default settings (Oasis II) and joining it on the other, I get a crash with the following error message (full crashlog.txt and application log are attached):

unhandled exception (PSERROR_Game_World_MapLoadFailed("Failed to load scenario.
Check application log for details."))
Location: unknown:0 (RtlInitializeExceptionChain)
Call stack:  (error while dumping stack: No stack frames found)
errno = 13 (Insufficient access rights to open file)
OS error = 0 (no error code was set) 

Application log shows this error:

ERROR: Failed to load scenario maps/scenarios/Oasis II.pmp: File_OpenFailed

This is on Windows Vista 32-bit Home Premium. Philip noted this on IRC:

01:23:09 ‹@Philip`› Windows doesn't like multiple people accessing the same files at once
01:23:22 ‹@Philip`› so you get errors when running two copies of the game
01:23:28 ‹@Philip`› since they're conflicting over cache files, I think
01:28:41 ‹@Philip`› I imagine we would have to do something like implement a fancy locking 
scheme when writing files to the cache, in case two processes are both writing
01:28:55 ‹@Philip`› which sounds not entirely trivial

Side note: this ticket may be related to #900.

[historic_bruno]

Another report involving Atlas and the game: #2534