They both come from france (改为同义句） ——— ——— ——— come from france

both of them
both可以有多种用法：
adj.两个的；两者的
adv.并；又；两者皆
pron.双方都；两者都
conj.既…且…
They both come from france 这里的both是副词,在句子中作为状语,表示“两者皆”
Both of them come from france.这里的both是代词,表示“双方都,两者都”
没什么理由,就是这么用