There is nothing wrong with two employees not liking each other, provided they conduct themselves in a professional manner.

For instance, it is very bad that Alice tells others that she does not like Bob. That is unprofessional and unacceptable.

Regardless of how they feel about each other, as a manager you need to have an expectation that they communicate effectively **when they need to**.

Your job isn't to get to the bottom of their dislike and act as a relationship counsellor. Two people cannot be made to like each other.

If you think the situation could lead to such problems, you can get on the front foot and indicate to them that they should make efforts to improve the working relationship, and that you won't tolerate unprofessional behaviour.

If one of them has a specific grievance, they should bring that up professionally with the other, and if that does not work, the correct course of action is for them to raise it with you. You should then act on the merits of that complaint and in line with your company's HR policy.