public class Tournament
{
    [Key]
    public string TournamentID { get; set; }
    public DateTime TournamentDate { get; set; }
    public string Place { get; set; }

    [ForeignKey("TeamA")]
    public string TeamAID { get; set; }
    public Team TeamA { get; set; }

    [ForeignKey("TeamB")]
    public string TeamBID { get; set; }
    public Team TeamB { get; set; }
}

public class Team
{
    [Key]
    public string TeamID { get; set; }
    public string TeamName { get; set; }
    public string Captain { get; set; }

    [InverseProperty("TeamA")]
    public virtual ICollection<Tournament> TeamA { get; set; }

    [InverseProperty("TeamB")]
    public virtual ICollection<Tournament> TeamB { get; set; }
}

public class TeamVM
{
    public string ID { get; set; }
    public string Name { get; set; }
    public string Captain { get; set; }
    public virtual IEnumerable<TournamentVM> Tournaments { get; set; }
}
public class TournamentVM
{
    public string ID { get; set; }
    public DateTime Date { get; set; }
    public string Place { get; set; }
    public string Competitor { get; set; }
}

public async Task<IActionResult> TheAction(string id)
{
    Team team = await _context.Teams.Where(x => x.TeamID == id).Include(x => x.TeamA).Include(x => x.TeamB).SingleOrDefaultAsync();

    TeamVM model = new TeamVM
    {
        ID = team.TeamID,
        Name = team.TeamName,
        Captain = team.Captain,
        Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
        {
            ID = x.TournamentID,
            Date = x.TournamentDate,
            Place = x.Place,
            Competitor = x.TeamB.TeamName
        }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
        {
            ID = x.TournamentID,
            Date = x.TournamentDate,
            Place = x.Place,
            Competitor = x.TeamA.TeamName
        })).OrderBy(x => x.Date)
    };
    return View(model);
}

Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamB.TeamName
    }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamA.TeamName
    })).OrderBy(x => x.Date)

Tournaments = team.TeamA.Where(x => x.TeamAID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamA.TeamName //i.e. TeamA instead of TeamB
    }).Concat(team.TeamB.Where(x => x.TeamBID == team.TeamID).Select(x => new TournamentVM
    {
        ID = x.TournamentID,
        Date = x.TournamentDate,
        Place = x.Place,
        Competitor = x.TeamB.TeamName //i.e. TeamB instead of TeamA
    })).OrderBy(x => x.Date)

@model TeamVM
<div>@Model.ID</div>
<div>@Model.Name</div>
<div>@Model.Captain</div>
<table>
    <thead>
        <tr>
            <th>ID</th>
            <th>Date</th>
            <th>Place</th>
            <th>Competitor</th>
        </tr>
    </thead>
    <tbody>
        @foreach (var item in Model.Tournaments)
        {
            <tr>
                <td>@item.ID</td>
                <td>@item.Date</td>
                <td>@item.Place</td>
                <td>@item.Competitor</td>
            </tr>
        }
    </tbody>
</table>