For a scalar BFB, you ought to do it when just one person can be in the field. The results of a BFB scan of two people simultaneously would be meaningless. It would be like taking a blood sample from each, mixing it together and sending it in for testing. Of what use would that be?
Maybe you're just asking about scanning while sleeping though. If that's the case, yes. Certainly, you could do a scan on a person while they are sleeping.
You're going to be applying a frequency to one person who needs it, and another who doesn't. While treating with frequencies is very safe, it could possibly have an adverse effect on someone who doesn't need it. It's pretty much impossible to be fatal, cause cancer or that sort of thing. But you can have a reaction to frequency where it can make you fell anything from a headache to nausea to flu-like symptoms. Those would be extreme. But I've had people have immediate negative reactions to frequencies which SHOULD have helped. Even following best practices, you can make someone feel less than great.
Ultimately, Scalar is pretty safe. So do what you want. But it's not a "best practice" to do biofeedback on two people at once and apply frequency to both.
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