I don't believe you can opt out for one practice location but not another. If you are opted-out then your locums agency can't bill Medicare in your name. If you are opted-in you are not allowed to charge more than the Medicare rate in your private practice.
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Can a physician or practitioner "opt out" for some Carrier jurisdictions but not others? No. The opt out applies to all items or services the physician or practitioner furnishes to Medicare beneficiaries, regardless of the location where such items or services are furnished."
https://www.sanctionscreeningnow.com/opt-out_faq.html
"“[W]hen a group physician has opted out, it does not affect the ability of the rest of the group members to furnish and bill for services they furnish to Medicare beneficiaries…. [H]owever, … when a group physician has opted out, the group may not bill in its own name for services provided by the opt-out physician under a private contract….” (66 FR 856, 1/4/01)."
http://aapsonline.org/notes-on-opting-out-of-medicare/
The best approach is just not to take Medicare patients in your private practice at all. Taking them and charging them more than the Medicare rate while you are simultaneously contracted with Medicare elsewhere is a big no-no.