Yes an SQP from one company can sign off from another company.
The SQP needs to have satisfied themselves of the competencies of those that have undertaken the work which they can obviously do through CVs, company skills frameworks, SoBRA accreditation scheme etc etc.
The important thing would be for the SQP ideally to be involved from the outset of the project to ensure that things are correct from the outset and guides what they consider is good practice. The risk of using someone from another company is that they may not agree with the conclusions as they have not been involved earlier enough.
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