What you don't want is ==DCC====|====DC== anywhere. Toggle switch or no toggle switch, it's too easy to overrun the gaps, and with cars with metal wheels, or a loco, this will connect the two systems and SOMETHING will fry.
A dead section in this case means that when the interchange is switched to DC< it looks somethign like this:
==DCC===|<no power>|===DC==|====DC=======
So that even if you do run past the gap, it won; link DC and DCC.
ANd when you finish with the DC loco and switch the interchange back to DCC, it looks liek this:
=====DCC====|==DCC==|===<dead>==|===DC===
where the | are gaps in both rails.
--Randy