Entity Framework : Sharing entities across different DbContexts

I'm developing a plugin application with EF6, code first.

I have one main context with an entity called User:

public class MainDataContext : DbContext
{
    public MainDataContext(): base("MainDataContextCS") {}
    public DbSet<User> Users { get; set; }
}

And then another context for PluginX, on another project which references the base one:

public class PluginDataContext : DbContext
{
    public PluginDataContext () : base("MainDataContextCS") {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        modelBuilder.HasDefaultSchema("PluginX");
        base.OnModelCreating(modelBuilder);
    }

    public DbSet<Booking> Bookings { get; set; }
}

And this neatly creates, on the same Database (same connection string), the PluginX.Bookings Table.

The problem here is that the Booking entity contains a reference to User entity:

public class Booking
{
    public int Id { get; set;}
    public virtual User CreationUser { get; set;}
    public BookingStatus Status { get; set; }
}

And when running Add-Migration for the plugin context EF will try to create another User entity called PluginX.User.

How can this be solved? Is there a way to share a common entity, in another DbContext?