This will actually depend on the couple; in my opinion, I think it has its advantages and disadvantages. First on the positive side, they will be able to help each other out on different projects or assignments at work. On the negative side, just in case one gets a higher rank or position, it may cause conflicts. Maybe the one on the higher rank has to set high standards of the company; in this case negative thoughts might hit the other partner.
For some couples it works, for most it wont. My wife and I have been married 12 yrs and we don't have the problems others do. We have worked at Lowes with each other for 3 years (we moved cities) and also at UPS for 7 months. We found out that she has physical problems after she joined UPS so she left that job. Both times we worked VERY closely ( within 20 feet) to each other. We kind of keep our distance from each other during actual work and talk during breaks or lunch. She has her work friends and I have mine. There are times that we don't talk at all during our shift! Kind of act like co-workers and not spouses.
If we are mad at the other then we don't bring that to work... AT ALL! We just stay away from the other. Well... I may tell my friend and she hers but we don't argue there. We are very much a team. WE DO NOT lie to each other or anyone, we know what the other is feeling or doing. Not controlling. It IS truly freeing.
We don't spend all of our free time together so I believe that is an important reason we get along well.
Best of luck to you! If you are best friends like us then it will be fine, however if you need a lot of space then it may not.