Answer: Hedge plantings are traditionally very tight, especially for a formally trimmed hedge, although in my experience planting a bit further apart is preferable. At this point I don't think you will be able to remove every other one, the roots will have become thoroughly entwined below ground so digging them out is not practical. Cutting them off short will simply cause them to regrow.
Photinias are subject to numerous pest and disease problems so close spacing may not really be the cause of the problem(s) you are seeing. I would suggest you work with your county extension to determine what is happening to them, then work from there. It is possible a hard pruning and good follow up care could rejuvenate it for you. I'm sorry I can't be more specific for you.
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