I'm writing about an OC's home empire for a short bio, and I am completely baffled as to what would happen when she marries her husband, who is the heir to the throne of a different empire. Would it just become one large empire?
As mentioned by Oldcat, marriages of state should be carefully negotiated beforehand. Assuming that didn't happen, consider this:
- The heir of the Empress of A and the Emperor of B might be in line for the throne of A and B. Are the rules of succession compatible? For instance, B might have a salic succession while A is strictly based on primogeniture. If the first child is a boy, he will inherit the crowns of A and B. A girl would only inherit in A.
- As mentioned by SJuan76, having the same head of state doesn't necessarily unify empires. The technical term is personal union, and it may or may not last. Consider that Elisabeth II is queen of the Bahamas, but that doesn't make the Bahamas part of the UK.
- How strong are laws and traditions in both empires? Can a strong ruler overcome them? The Habsburgs tried that, and unrest followed.
Summarized, empires don't merge just because their current rulers are married. On the other hand, that is one way how mergers can happen.