Christian,

Same subnet - No

Same Vlan - Yes although not advised

Each interface on the PIX must be in a unique subnet. You can however connect multiple interfaces to the same backend VLAN but I don't know why you would really want to. To be ultra secure, each interface on the primary PIX *should* connect to the stand-by PIX via a seperate switch. In some cases, this is not feasible however so creating seperate VLAN's on 1 or 2 physical switches will also work.

Hope this makes sense.

Scott