DDD - How Can I Avoid Crossing Aggregate Boundaries Here?

We're working on a new project (re-writing existing app), and I'm running into problems with my domain model / repository design.

Here is a (simplified) version of two key portions in our domain model:

As you can see, I have an abstract concept of a Post, which can be things like a Review, a Discussion, a Photo, a Video, etc. Posts can also have comments.

I also have a abstract concept of a Location, which are obviously things like Streets, Cities, Neighbourhoods, etc.

Now, this naturally looked to me as two clear aggregate roots.

So I created two repositories, one called PostRepository, and another called LocationRepository.

This was all working fine, I can add/get any type of Post (or comment), and add/get any type of Location via one of these two repositories.

But now im in the scenario of a "landing page" for a City (for example).

On this page, I need to basically show "all posts for this location".

How is that defined? Well, a Post can be (optionally) tagged at a Location. Implementation detail, so I don't want to go too deep into data (as that's not what DDD is about), but essentially there is geospatial intelligence to work out which posts are contained in a particular location by the shape file of the location, and the latitude/longitude of the tagged Post.

But how can I retrieve this information without crossing the boundaries?

Which repository do I use? Do I need a new one?

If it matters (or for the curious), this is a web application (ASP.NET MVC), with a SQL Server 2008 database and Entity Framework 4.0.

If you need any clarification, let me know.

EDIT

We currently use a modified version of the Specification pattern in order to retrieve domain models.

For example, this is the code in our BLL to retrieve all Review's where Score >= 4:

var reviews = postRepository // GenericRepository<Post>
      .Find() // IQueryable<Post>
      .OfType<Review>() // IQueryable<Review>
      .Where(x => x.Score >= 4)
      .ToList(); // List<Review>

But now I need some code like this:

var reviews = postRepository
    .Find()
    .OfType<Review>()
    .Where( //lat long, or Locations FK )
    .ToList();

The problem is I don't know how to do the above query without adding an intermediary join-entity (LocationPost - as it's a many to many), and add a FK to the Post domain model to that.

But by doing that, I am crossing the aggregate boundaries - aren't I?