Because Jesus gives leaders to his people, he expects his people to submit to his leaders. And that just makes the whole dual church membership really hard to work out in practice.
How do you truly follow two groups of leaders? How are you really submitting to one church leadership group, if you’re going there on Sunday, but going to a different church small group during the week? Do the small group leaders have authority over you? If they do, doesn’t that mean their leaders will be held responsible for you too? But if you don’t go to that church… do those leaders even know you?
In the end, you can’t submit to two leaders equally. It’s like God and money, you’ll end up getting annoyed at one of them. You have to have a primary and a secondary.