Multiple users and households in Sims 3

Solution 1:

Unlike in the Sims 2, each save file exists in its own, separate universe.

In the Sims 2, time did not pass for sims you weren't playing. However, in the Sims 3, the "Story Progression" means that no matter which household you play, time will pass for all families in the neighborhood.

The one thing that is shared between neighborhoods is the Family bins -- that is to say, both you and your fiancee should have each other's sim in their neighborhood bin. If you then move this sim into the neighborhood (even if you don't change the active household), you will be able to find, meet, seduce, and eventually marry each other's sim.

However, because you have separate save files, marrying in one neighborhood won't simultaneously marry the sims in the second neighborhood.

It is possible to simply play multiple active families in the same neighborhood, but since every household ages at the same rate in The Sims 3, this will probably not be a preferable way to play (not to mention losing wants every time you switch active households).

Solution 2:

If you want to marry your sim to your fiancee's sim, you can click on Edit Town, there you'll see family bin, select your fiancee's sim, move a copy to your sim's town. That way you can have her in your own town(world), your sim can look her up using Online Dating on computer, and increase the relationship value with her until they both agree to marry.