We use completely separate (non-yoked) parallel breakers all the time for redundancy. Example: Parallel-Redundant UPS system. Also, N+1 generators: each gen has its own output breaker. This is all allowed under NEC and works well in practice, too. EXTREMELY well, too, as in 99.999-99.9999% reliability well. In the case of the generators, they have integrated governer and droop-adjusting circuitry to maintain proportionate load, but the UPSs maintain even load, even in bypass. In a properly cabled system, minute differences in breaker manufacture and cable installation don't amount to much of a current split difference. And if one breaker should trip while the other somehow miraculously doesn't? Alls the better, as you have time to fix the first UPS!
Care has to be taken with interlocks (both key-type and PLC) when multiple switchboards are used, to prevent the branch circuit from becoming a bus tie, but otherwise, it's pretty straightforward.
[This message has been edited by SteveFehr (edited 07-03-2006).]
