Why does the Yak-28 cockpit have two attitude indicators?
It's possible that the soviet attitude indicators are subject to tumble when operated inverted. the easy way to overcome this is to provide two instruments, one of which does not tumble when flying mostly right-side up and the other does the same for flying upside-down. 

Soviet-era acrobatic planes had dual G-meters installed for this same reason. One was installed upside-down and registered negative G's while the other was installed right side-up and registered positive G's.