It's best to create two save games close to each other. Log into one and fly to the homeworld of the other saved game. Other saved games are marked by the rotating blue lines. Forging an alliance should be pretty simple (especially on "easy"). You won't get the achievement right off, because you have to next log in on the other game and do it all over again. Once both games have forged alliances with each other, you will get the achievement.
Choosing "My Creations" as your game's theme greatly increases the chances of meeting your creatures to the point other users' creations become rare.
If you create a lot outside of the main game, especially Empire creatures, you are much more likely to get this achievement.
Another way to get this achievement is to place Monoliths, promoting two creatures that you created to Space stage and ally with them both. If you are unable to find two of your own creations, you can use the Creature Tweaker to create them.
Allying with one of your creations, allowing the alliance to be severed, and then re-allying them also works (this only works if you have installed Patch 3).