I know of a situation where both alternators failed. It was belt driven alternators on a twin, one on each engine. The first belt broke, taking that alternator. The remaining alternator's load doubled and that belt subsequently broke as well. Night IFR in icing conditions too. Prop heat and windscreen hear are big electric hogs. They were already on approach, and landed in about 5 minutes. Very lucky because the battery ran out in 6 minutes, before they even got to the after landing step of turning off the deicing systems.
The other time was after a lightning strike, but they were generators again. The old "turn it off and back on again" trick restored both generators.
We typically have a gear driven secondary alternator, so no belt to break. However I could see that situation happen in reverse. Backup alternator fails for any other reason (regator, brushes, etc). Primary alternator takes the extra load and breaks the belt.
The other failure mode to consider is a short circuit on a bus. A screw rattles loose on something and lands across a big connector or on the bus. Shorts the whole bus to ground. Doesn't matter how many alternators feed the bus, it's going to stop working. This is an extremely rare failure mode, and most small aircraft don't even consider it in the design. Not sure we need to either.
I'm leaning towards full EFI, so need decent electrical redundancy. The best there is will be dual independent electrical systems. Two alternators, two batteries, two busses. Keep them independent with no cross ties or anything. Do whatever failure on one and the other is unaffected. Putting one EFI, one fuel pump and the taxi light on the smaller bus. The bigger bus gets to start the engine and run everything else. Heated seats, avionics, flaps, etc. The backup bus only needs a very small, lightweight battery since it's not starting an engine or requiring endurance. I suppose it load sheds in the sense that if it's not super important (engine computer), I only have one. After a complete failure of one electrical system, there is one EFI computer and one fuel pump still working.
A multi screen IFR panel could feed the backup avionics from the smaller bus.