You get better isolation when the modules are placed like:
1. The E planes of the antenna are orthogonal to each other,
2. As far from each other as possible (farther the better, no limit to it).
3. Place one module at a direction where the other module has its null gain(or minimum gain).
So, we need to get the radiation pattern details for this.
Could you please let me know why you are using two modules in the same product. If we know that, we can see if there are other alternates.