Yes, this might work. I'am not sure if you might get problems with quota and file permissions if you dont sync /etc/passwd and /etc/shadow between the two servers.
Didn't think about that.
If a fileserver doesn't make it in v.3, then it certainly will in v.4 .
If you can do a gateway/router service too, ispconfig will be more like a 'total server solution'.
Veni, vidi, Foetsie
(I came, I saw, I was gone