There's a "oops" scenario here. Say you shut off the truck and leave your lights on. Come back later and red battery is dead, but with enough juice to engage the solenoid when the key is turned on. Even though the red battery is still dead and incapable of starting the truck, the yellow battery provides voltage to all circuits (except starter) before you hit 'start' so you don't know red battery is dead. But when you engage start, starting current will blow yellow fuse. It's not such dangerous failure mode as long as the fuse blows.
The way around this is having a smarter solenoid that only engages at over 12v. That way the yellow battery begins charging only when alternator is charging so you can't try to start with it.
1988 U-1300L Unimog with 10' Alaskan camper, 1994 LMTV soon to have Alaskan 10' NCO camper, 1957 404 Mog, 1965 404 Mog, 1970 404 Mog, 1970 406 mog, 1972 406 Mog, 1978 406 Mog, 1968 Haflinger, 1952 Willys.