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If your processor has the 5.1 Outs at signal level (phono plugs, not speaker cable connectors), then perhaps you could connect the centre out to your left-in on the 2nd stereo amp.
Then plug a phono from the left-hand phono line-out of you stereo amp to the left input on it, effectively getting left and right to have the same signal.
I use this method for my subs in my car , but to be honest, you're probably best off not going this route as all these ins & outs will probably mean no benefit due to the cables.
Check with other members to see if this is safe. Don't try until confirm: