可以。先说who,主语从句：Who killed the man is still a mystery(谜团)。这是一个主语从句。同位语从句：The question who will take his place is still not clear. （who作主语），是主语的同位语。再说how,主语从句：How he killed the man is still a mystery(谜团).这是一个主语从句。同位语从句：The question how he killed the man is still not clear.（who作主语），是主语的同位语。
