Binocular refraction should not be used where acuities are markedly unequal, or one eye is strongly "dominant".
If you do start to apply it to an unsuitable patient, they will usually tell you fairly quickly ("Should I be seeing double?").
In such cases occlude the better eye, refract the worst eye.
Then refract the better eye with the worst, eye fogged, if necessary, or even unfogged if the acuity is considerably worse than the eye you are going to test.
If the worst eye is 6/9 or less, it may be unnecessary to fog it.
If the difference is less it is often possible to refract binocularly, but the final decision has to be taken on an individual case basis.