Err, it hasn't happened yet, but I think this time it will.
It worked adding one all-star to complete a team. Detroit added Rasheed Wallace and won a Championship. Philadelphia added Moses Malone to a team that had already been to the finals, but couldn't win it, and they won with Moses.
It's simply rare that a team is in a position to go after two All-Star players, so it hasn't been a common occurrence. Without a large sample, it's hard to answer the question authoritatively, but I would speculate yes.
If you add the right pieces together, regardless of how they're assembled, you can build a team that can contend for a Championship. There is never a guarantee that you will succeed.