Question

He has two sons,both of whom work as doctors .

其中先行词是什么，为什么这里both of 放在先行词whom前面？

Answer 1

因为先行词（ 关系代词whom）在从句中做介词of的宾语。如： He has two sons, both of whom are workers. He has two sons and both of them are workers.

Answer 2

who的宾格形式是whom，放在介词of后面，没有什么问题啊。

追问

但一般来说都是介词+先行词结构，为什么有both？

Answer 3

不可。如果改成them，那么both of them are teachers就是一句完整句子，一个逗号是不可连接两个句子的，如果改为He has two sons and both of them are teachers.就可以了。