成唯識論述記 7.383

7.383

述曰：一有情身一無間緣，如何俱時有多識轉？不爾，一念多無間緣。

The Commentary states: In the body of a single sentient being, there is only one equal-and-immediate condition; how can multiple consciousnesses operate simultaneously? Otherwise, in a single moment there would be multiple equal-and-immediate conditions.