To determine whether the chip is in single or dual plane mode, you should read F3xA0.
If that bit is 1, the chip is operating in single plane mode and if it is 0, VDD and VDDNB
are controlled separately through both the SVI and the PVI interfaces (the latter
operating as an SVI). See section 184.108.40.206 of the BKDG. The rest of the section 2.4.2 is
interesting reading as well.