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My question is can this be setup as a singe Stack Power group so that the 715w supplies in the WS-C3750X-48P-E can be used as backups for the 350w supplies in the fiber WS-C3750X-12S-E?
It depends on whether you'll have enough excess power in the stack to make up for the power loss if a 350W PS fails (don't forget the converse, i.e. if a 715W PS fails) and whether you would exceed the power that the StackPower cables permit (2000W). Regarding making up power, you do have the option to prioritize power usage, i.e. "automatically" shut power to lower priority devices, to maintain power for your critical usage.
Ideally you want to minimize the need for power to cross the StackPower, either normally, or if there's a PS failure. This is accomplished by how you place (power balance) your PSs in the stack. For example, if your 5th PS is another 715W, and your PoE usage isn't real high on either PoE switch, you might move one of the 315W PSs to the other 12S or one of the 48Ps and place the "extra" 715W in the 12S where you removed the 315W.
