Здравствуйте, Yuri Abele, Вы писали:

YA>var entityOne = this.DbContext.EntiesOne.SingleAsync(e1 => e1 == 123);
YA>var entitiesTwo = this.DbContext.EntiesTwo.Where(e2 => e2.Abcd == entityOne.Abcd).ToListAsync();

У вас именно так написано или все таки стоит await:

var entityOne = await this.DbContext.EntiesOne.SingleAsync(e1 => e1 == 123);
var entitiesTwo = await this.DbContext.EntiesTwo.Where(e2 => e2.Abcd == entityOne.Abcd).ToListAsync();

Из справки:
Remarks
Multiple active operations on the same context instance are not supported. Use 'await' to ensure
that any asynchronous operations have completed before calling another method on this context.