Currently when you form an alliance with a civilization the enemies of that civilization say they don't like it including civilizations that you are allied with. So you can be allied with civilizations that are enemies of each other. This can be unrealistic in some cases.
To make it more realistic some civs should give you an ultimatum - either ally with me or ally with the other civ but you can't be allied with both at the same time. You'll have to make a choice to support one civ or group of civs, or choose another civ or group of civs.
To make it more realistic some civs should give you an ultimatum - either ally with me or ally with the other civ but you can't be allied with both at the same time. You'll have to make a choice to support one civ or group of civs, or choose another civ or group of civs.