一句普通的英文。

This is Jim and Tom's room. 只有一间房间，所以是两人公用！
These are Jim's and Tom's rooms. 两人各自的房间

两个人的房间怎么能用单数呢。如果两个人共用一个房间，那也只能说jim and tom's room

第二个回答的对

This is Jim and Tom's room. (两个共同住的房间）
These are Jim's and Tom's rooms.(两个人分别有的房间）

很明显is,room表明单数,说明房间只有一个,是两个人共同的房间,所以该this is jim and tom's room