Leave room inside for a small fire resistant lock box for small valuables and important paperwork. The heat will have to breech the big safe, then that one. It'll buy you some time. You might not be home when the fire starts.
Remember that "fire-safe" actually means fire resistant, based on time and temperature limits. When my part of Bastrop county went up in flames in 2011, even the very, very expensive safes didn't protect the contents. The fire guys just couldn't battle so many fires at once - so 1,700 homes were allowed to burn themselves out over 10-12 days. Safes are rated in minutes in high temperature - not days.......
One of my neighbors in particular, didn't look so good when the pro's finally got his large and very expensive safe open. (Interesting process) Massive amounts of cash, jewelry, guns and rare items evaporated all over the hills that week.
Ever see four pools of solidified liquid aluminum pools next to a car, then realize that it was the rims?
One is none - two is one. How important is "important".......