A split bus is normally operated as a single bus. Power placed on any point on the bus powers the whole bus. If the bus is AC, then any power source placed on the bus needs to be regulated in voltage, frequency, and phase. When you say "generator failure," you're not specific as to the nature of the failure. It's possible to have a generator problem, with a generator that can still serve it's respective bus, but won't parallel with other generators on the main bus; in this case the bus can be split, or the generator isolated. It's failed in the sense that it doesn't play well with other generators, but is still performing a useful job. Where the split system breaker or bus tie is normally closed, it may be opened for a fault, or to isolate each generator from other generators, or from other faults.
So long as DC voltages match, you're golden. You can put them together. AC is another animal, and has more requirements.