Set inherited permissions on iLifeAssetManagement?
I would try to avoid ACL's: they are tricky and often behave unexpectedly.

If I were you, I would try stick to standard UNIX permission, following these guidelines:


Create a user group (say, MeAndWife)
Make sure users Me and Wife are the ones and only members of MeAndWife
Change group ownership of sub directory to MeAndWife by something like chgrp -hR MeAndWife '/Users/Wife/Library/Application Support/iLifeAssetManagement/assets/sub'
Grant the owning group members the same permissions which are granted to the owning user: this may be obtained in several ways, but one that I like is by running find '/Users/Wife/Library/Application Support/iLifeAssetManagement/assets/sub' -group MeAndWife -and -perm -u=r -and -not -perm -g=r -exec chmod -h g+r {} \; [this basically states: Find files/folders with owning group MeAndWife, for which owning user does have read permission, but owning group members do not; and give owning group members read permissions]. Of course you should repeat this step for write and execute permissions (just replace -u=r and -g=r with, respectively, -u=w and -g=w for write, -u=x and -g=x for execute).
Finally, I think that if you want to be able to navigate to the sub directory from your account, then you need to make the full path to your sub directory searchable by everyone. That means, run through chmod o+x '/Users/Wife/Library', chmod o+x '/Users/Wife/Library/Application Support', chmod '/Users/Wife/Library/Application Support/iLifeAssetManagement' and chmod o+x '/Users/Wife/Library/Application Support/iLifeAssetManagement/assets'


That should do the job: new files in sub should inherit group ownership (MeAndWife) and permissions (same as owning user). If you find that that is not the case, you could use launchd in conjuction with a very simple script to fix file permissions periodically (say, hourly).