EF models. Navigation properties can only participate in a single relationship

You can't reuse Team.Games as the inverse property for both Game.FirstTeam and Team.SecondTeam. Think of it, if you add game to Team.Games, how would EF know which team it is, first or second?

You need two collections to describe the relationships. And that's also a chance to add some more meaning to the class model. For example (only modified code):

public class Game
{
    ...
    public int HomeTeamId { get; set; }
    public Team HomeTeam { get; set; }
    public int AwayTeamId { get; set; }
    public Team AwayTeam { get; set; }
}

public class Team
{
    ...
    public List<Game> HomeGames { get; set; }
    public List<Game> AwayGames { get; set; }
}

For a team it's meaningful to make a distinction between home and away games, for example to compare results in both types of games.

And the mapping:

modelBuilder.Entity<Game>()
    .HasOne(g => g.HomeTeam)
    .WithMany(t => t.HomeGames)
    .HasForeignKey(t => t.HomeTeamId)
    .HasPrincipalKey(t => t.Id);
modelBuilder.Entity<Game>()
    .HasOne(g => g.AwayTeam)
    .WithMany(t => t.AwayGames)
    .HasForeignKey(t => t.AwayTeamId).OnDelete(DeleteBehavior.NoAction)
    .HasPrincipalKey(t => t.Id);

If using Sql Server, this delete behavior instruction is necessary to prevent disallowed multiple cascade paths.

The problem is that your Team model has 2 one-to-many relationships with your Game model but you only have one navigation property on the Team.

You need to have 2 navigation properties on the Team model, one for each relationship. (Game1, Game2...).

You will also need to define these relationships in the Game model - a Team property for each relationship.

Check this answer for extra info.