Mm, u can do this:
1* Install Interlude Client
2* Change the name of the folder "Lineage II" to "Lineage II Interlude"
3* Install Gracia Part II Client,
or
2'* Copy the folder called "Lineage II", and the change the name from "Copy of Lineage II" to "Lineage II Interlude",
3'* Now go to Lineage II>Lineage2.exe and do the Update
Now, u have both clients:
Folder "Lineage II" > Gracia Part II
Folder "Lineage II Interlude" > Interlude