I am not very familiar with SH5 multi-missions, but in SH4, you need a 2nd computer and a network "session" with it to test your multi-missions. You don't need another player, just a computer with SH5 on it running in multi-player that is nearby that you can log into the same session as the other computer with. SH4 does not let just one person play a multi-mission, which can be very frustrating when you are wanting to get it set just so...
