How do I use EntityDatasource (Or LinqDatasource) with a Union clause

How do I combine records from two tables in an Entitydatasource control? I have googled it and searched on SO with no luck. The SQL of what I need is

SELECT DISTINCT username FROM 
(SELECT s.username FROM project_stakeholders s
UNION
SELECT t.username FROM project_team_members t)

My entities structure is as follows:

project_stakeholders
----------------------
project_stakeholders.record_id (PK)
project_stakeholders.username
project_stakeholders.project
project_stakeholders.project_id (FK)
project_stakeholders.status

project_team_members
---------------------
project_team_members.record_id (PK)
project_team_members.username
project_team_members.project
project_team_members.project_id (FK)
project_team_members.status

I found this question that almost provides an answer to my question but mine is more advanced because my SubQueries are from Entities not related to the primary entity.

I have changed my Datasource to LiqDatasource Control and added the code below in the Code Behind of the Onselecting Event of the LinqDatasource Control:

    protected void TaskProfileDS_Selecting(object sender, LinqDataSourceSelectEventArgs e)
    {

        int project_id = 0;

        if (Int32.TryParse(Request.QueryString["id"], out project_id))
        {

            using (MyEntitiesContext db = new MyEntitiesContext())
            {
                var TeamMembers = from tm in db.team_members where tm.project_id == project_id select tm.username;
                var Stakeholders = from sh in db.project_stakeholders where sh.project_id == project_id select sh.username;
                var result = from usr in db.project_profiles where (TeamMembers.Contains(usr.caption) || Stakeholders.Contains(usr.caption)) select usr;

                e.Result = result;

            }
        }
    }

But unfortunately, I get the following error:

The operation cannot be completed because the DbContext has been disposed.

any ideas will be greatly appreciated.

Here is the code that worked for me to fetch records from unrelated tables (entities). I used LinqDataSource, then in the QueryCreated event, I changed the query to my own LINQ query:

protected void TaskDS_QueryCreated(object sender, QueryCreatedEventArgs e)
    {
        int project_id = 0;

        if (Int32.TryParse(Request.QueryString["id"], out project_id))
        {

            using (SDManagerEntities db = new SDManagerEntities())
            {
                List<project_profiles> usrs = new List<project_profiles>();

                var users1 = db.team_members.Where(t => t.project_id == project_id).Select(it => it.username);
                var users2 = db.project_stakeholders.Where(s => s.project_id == project_id).Select(it => it.username);

                var allMembers = users1.Concat(users2);

                foreach (var member in allMembers.Distinct())
                {
                    project_profiles usr = db.project_profiles.SingleOrDefault(it => it.caption == member);
                    if (!usrs.Contains(usr))
                    {
                        usrs.Add(usr);
                    }
                }

                e.Query = usrs.AsQueryable().OrderBy(it => it.fullName);

            }
        }
    }

I hope it helps someone else