Here is an example (going from memory):
MAP1
info_landmark
name: hello
trigger_changelevel
new map name: map2
landmark name: hello
MAP2
info_landmark
name: hello
trigger_changelevel
new map name: map1
landmark name: hello
Also, your not allowed to have capitals or spaces in the new map name feild, so just keep things simple, other than that, should work.
Aaron's Stuff