iTunes home sharing, between 2 iTunes accounts, but 2 way? The situation is that I have an iTunes library and account and my girlfriend also has an iTunes library and account in her name (on a separate machine).

While there is some overlap in our libraries we would like to be able to add music from the other's library to our own, can iTunes home sharing be used for this? Whilst maintaining separate iTunes accounts?

(I have only ever used home sharing to sync 2 libraries using the same iTunes account)
You can use separate iTunes accounts, but the HomeSharing account must be the same. You will need to authorize each other's computers/devices with both iTunes accounts in order to play back content purchased under the other account.