know their right hand from their left

具体语境是啥

字面意思我认为是知道从左手到右手里的东西

找了段文章
Do you know your right hand from your left? Of course you do – but a surprisingly large number of doctors don’t. This can be a problem if you’re amputating a leg.

Sometimes they get the right and left bit sorted out, but still carry out the wrong procedure on the patient, or they get the procedure right, but on a patient who should never had it in the first place.

这里的意思就是说你知道你在做什么吗？你当然知道——但是让人吃惊的是大多数的医生并不知道自己再做什么，如果你要做截肢手术的话这就成为了一个大麻烦。

知道他们的右手从左边

Sometimes they get the right and left bit sorted out, but still carry out the wrong procedure on the patient, or they get the procedure right, but on a patient who should never had it in the first place.

有的时侯他们会吧正确的左右位置整理好，但这仍然在执行病人的错误做法，又或者他们了解了正确的做法，但是在病人之中谁将经历这第一次安排