Usually they only work if the two regiments in questions already socialize. That is to say, perhaps they're in a coalition together and their members play together, etc. This makes members feel much more comfortable around each other. The opposite is true when the two regiments' members don't know each other and can end in animosity. I would recommend working together for a few line battles first and encouraging your members to play together first. If all goes well for a few weeks and everyone is comfortable with each other and their new potential officers, then a merge will likely succeed.