You can run the game in "multiplayer" simply by sharing the files of your fallout savegame and loading from the same savegame. When leaving a map (or saving), it puts the "updated" data on the disk, so when the "other" player reaches the same map you just parted from he has the result of what you did (say, killed the entire city). Of course this can cause a whole slew of problems since there is some data which is unique (ie: chracter stats and global variables) and thus can't be different for "both" players... The game will use the latest save's values for that, so we can't really do that. Not to mention we can desynch the internal values and the GVARs with the rest of the stuff...
As for IanOut, well, it all depends on the programmer.