I don't know about the "read only" aspect of a dual bios ...
... but a good many of the ASUS ROG boards have had the dual bios chips since Z68/M4EZ (not sure about before that) ...
... & pretty sure there are some of ASRock boards that have the 2 bios chips as well ...
It's a great feature to have on a board for testing new bios versions ...
... & the ability to flashback to an earlier bios version is very kool as well ...