you can use bluetooth and send files using the built in file manager's 'share' option.

if you really need to use wifi, you can check out this thread

http://talk.maemo.org/showthread.php?t=69912

it allows one n900 to act as a wifi hotspot and share a folder via samba. the other n900 would need to be able to mount samba shares though. there is an app called wizard mounter but i'm not sure it actually works. personally i use easy debian to mount samba shares but this is overkill