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Two separate optocouplers don't work well because it's effectively impossible to get identical characteristics. Problem in simulation is that gain variation isn't modelled unless you introduce in explicitely. IL300 is solving the problem by using two matched photodiodes in package. The matching isn't perfect however and still needs individual gain adjustment. Analog optocouplers are a last centuries technology if you're looking for precision, today's analog isolators are digital (AD/DA).

Once again regarding post #2 question. To transmit AC with analog isolator, you need to bias the LED current to midrange. An AC input optocoupler like ACPL-214 is no solution.