You kindof answered your own question... Running RO in parallel will produce twice the amount of product water and twice the amount of waste as opposed to a single membrane (while using twice the amount of water as a single membrane). So it will produce water twice as fast but use twice as much water. Running it in series is where you get twice the product and half the waste but as you said you have 350 TDS coming in so it might not be worth it to run them in series