At glide slope capture the 2 DC busses are automatically isolated from each other by the cross bus tie relay opening.
As geo said, you need two completely isolated electrical systems, each with its own autopilot, receivers etc for redundancy and for cross monitoring of signal and command input integrity.
It's not about
completely isolated electrical systems,
, it is about redundancy. The cross bus tie relay opening is a red herring in this case. The DC busses will be just as isolated with one generator supplying the power to both transfer busses as they are with two. However, there will be no redundancy should that generator fail, that is the issue.