Yes, an SGPV from one company can sign off from another company.
The SGPV 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, other industry accreditation scheme etc .
The important thing would be for the SGPV 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.