这种写法对吗??

有两个谓语咯
That Japanese eat sushi is common knowledge. 就对咯~~

上面的答案听起来别扭。我在美国长大，所以你可以相信我 ：）

这样说：

It is common knowledge that the Japanese eat sushi.

It's common knowledge for Japanese to eat sushi.
你将两个句子混合到一句了，只要把其中一个作为主句，另一个为补语就可以了。或者断开，
Japanese eat sushi，it is common knowledge.