Formally, the Department of State only recognizes tandems with other Foreign Service Officers, whether from DOS, USAID, USDA, or Commerce. FSOs with spouses in other government agencies just have to try to work their respective agency assignment processes to end up at the same place.
I don't know how the FAO assignment process works and how flexible it is, but as a new FSO, you will bid off of a bid list with about as many posts as there are people in your orientation class. You can certainly express your desire to be posted at the same post as your husband, but it depends on whether or not there's a position at his post on your bid list, whether or not the timing works, the needs of other people in your orientation class, and whether there's any other complicating factors.
For example, if your husband is going to Brasilia and Brasilia isn't on your list, you are out of luck. If Brasilia is on your list, the timing might not work. They might need someone who speaks Portuguese now and can't leave the job empty for the six months it would take you to learn Portuguese. There are fewer FSOs than military musicians, so leaving a job empty for six months could make life a lot harder at that post. If there's someone else in your orientation class that also needs Brasilia for some reason, then the assignment officers may be in a tough spot and have to figure out which one of you to send, assuming no other issues. Finally, there could be a complicating factor, like a medical clearance issue that might prevent you from being assigned to Brasilia.
So, there are no guarantees of assignments together. You'd have to both work your respective processes to try to get posted together or at nearby posts. The Career Development Officers are reasonable people, but they are bound by the vacancies that are available and also have to treat all FSOs equally, so they can't give priority to one based on where a spouse is posted. Even FSO-FSO tandems are not absolutely guaranteed assignments together or to get to post and leave post at the same time. There are some procedures in place to accommodate them, but it's not a guarantee.