How to build a two processor system?

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However, the tasks rely on RTOS would be much slower, it uses the RTOS system resources, the remaining resources would be wasted.

So the multiprocessor distributes system resource, would be much faster.

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It depends what kind of resource optimization you are interested in. If you would like to save the precious resources on FPGA, then obviously two processors together would consume more resources compared to one. In this case, one would wonder as to why he should use many processors while he can achieve nearly-the-same result with just one, thereby also saving FPGA resources.

However, if you have a crunching restraint on the design such that the task must be finished in a very small period, probably a multi-processor system would be useful.

I would put it as try-and-watch.