WIPIn game, go to singleplayer -> matches.
Make player 1 and player 2 allies, give player 1 The Athenians civ and player 2 the Carthaginians Civ.
Ingame in chat enter 'gift from the gods' cheat.
With ALT+D you can open dev console, you'll see a change perspective option, click it and next to the menu button on top right side you can change to player 2.
Make a foundation for an Embassy building (but don't build it). Now change perspective back to player 1 and verify that player 1 is not able to start building that building.