前面那句的语序不对。后面那句是正确的。

1. 他们工作在一起。 Wrong sentence order: “S + 在一起 + V + O” (It's a common mistake some learners make when they translate） 2. 他们在一起工作。 ( Correct ) Hope it helps :)

“他们在一起工作” is better grammatically. "他们工作在一起" is also OK, especially if one wants to stress the "在一起" at the end, like "他们吃饭在一起，睡觉在一起，工作也在一起". It can also be seen as that the “的” is omitted comparing to the original sentence “他们的工作在一起”.

they are just like "they are together working"and "they are working together"in english.

The second one sounds slightly more natural to me. But I also don't see any reason why the first one can't be used. The only difference I would say is that the first one kind of places the emphasis on the state of "being together" as opposed to the other sentence which could be purely a statement in a story without any emphasis.