How to map a non nullable foreign key that exists in two joined tables

I'm mapping a set of membership classes for my application using Fluent NHibernate. I'm mapping the classes to the asp.net membership database structure. The database schema relevant to the problem looks like this:

ASPNET_USERS
UserId        PK
ApplicationId FK NOT NULL
other user columns ...

ASPNET_MEMBERSHIP
UserId        PK,FK
ApplicationID FK NOT NULL
other membership columns...

There is a one to one relationship between these two tables. I'm attempting to join the two tables together and map data from both tables to a single 'User' entity which looks like this:

public class User
{
    public virtual Guid Id { get; set; }
    public virtual Guid ApplicationId { get; set; }

    // other properties to be mapped from aspnetuser/membership tables ...

My mapping file is as follows:

public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        Table("aspnet_Users");
        Id(user => user.Id).Column("UserId").GeneratedBy.GuidComb();
        Map(user => user.ApplicationId);
        // other user mappings

        Join("aspnet_Membership", join => {
            join.KeyColumn("UserId");
            join.Map(user => user.ApplicationId);
            // Map other things from membership to 'User' class
        }
    }
}

If I try to run with the code above I get a FluentConfiguration exception:
Tried to add property 'ApplicationId' when already added.

If I remove the line "Map(user => user.ApplicationId);" or change it to "Map(user => user.ApplicationId).Not.Update().Not.Insert();" then the application runs but I get the following exception when trying to insert a new user:
Cannot insert the value NULL into column 'ApplicationId', table 'ASPNETUsers_Dev.dbo.aspnet_Users'; column does not allow nulls. INSERT fails. The statement has been terminated.

If I leave the .Map(user => user.ApplicationId) as it originally was and make either of those changes to the join.Map(user => user.ApplicationId) then I get the same exception above except of course the exception is related to an insert into the aspnet_Membership table

So... how do I do this kind of mapping assuming I can't change my database schema?