[Endoperez]

There's no sharing, at all. You can add a command to a map file that tells the AI to not attack you (although it still might start attacking if you attack it first), but it won't do anything else. It won't send you gifts or gold or gems. You can simulate it by taking Luck 3. Can't go through his provinces either, and being caught sneaking counts as the scout attacking the province (if there is a fight).

Thankfully, you can also use the map editor commands to set the AI god, so you can e.g. set your dominion strengths to similar values so they spread about as fast. Keep in mind that scales only give benefits when province owner has dominion in the province, so spreading your dominion into enemy lands could be problematic. This is worse with Luck, which counts as NEGATIVE in enemy dominion - your luck is his misfortune.

Find doc folder in Dominions 3, and open mapedit.pdf. It describes the commandss available, including this:

2.4.8 #allies
These two players will not attack each other. This
command will only affect computer players.

The list of nation numbers is in the modding pdf, probably at the very end.

Map-editing is relatively easy. Just add your own commands to the end of existing map file without removing the original files, and start a new game on the map. DON'T use the map editor, it can't add these kinds of commands and at least used to delete some map commands. You should keep an unedited copy somewhere, just in case something goes wrong, but at worst you can always redownload/install/patch to get the original map back. You don't have to change the image, only the textfile with the .map ending.