observe.ts

   let o1:Observable<boolean> = this.cb1.valueChanges;
   let o2:Observable<boolean> = this.cb2.valueChanges;

   merge(o1, o2).subscribe( v=>{
   this.columnDefinitions[0].hide = this.cb1.value;
   this.columnDefinitions[1].hide = this.cb2.value;  
      console.log(this.columnDefinitions);
    });
  }