Since Photo Mechanic does not pair up tiff and jpeg files, the code replacements will not update both files at the sametime. There is an easy solution to your workflow. Once you have updated the IPTC metadata in one set of files, you can use the Text Exporter utility, with the Exp_Custom_Set_Xfer_Default Snapshot, to create a code replacement file. Add the Code replacement file to the Set Code Replacement utility and then apply the Metadata (IPTC) Template, with the MDT_Custom_Set_Xfer_default Snapshot, to load the matching code replacement variables. Apply the Metadata (IPTC) Template to the other set of files. I have attached the PM6_Metadata_Snapshots_3-23.pmx file with the Text Exporter and Metadata (IPTC) Template Snapshots for this workflow. The PMX file will be Imported into Photo Mechanic's preferences. If you don't want to Import all of the Snapshots from the PMX file then just select the Exp_Custom_Set_Xfer_Default and MDT_Custom_Set_Xfer_Default files from the Import list. I have also attached a text file, MD_Xfer_workflow.txt, with the step by step instructions for this workflow.
Best regards,
--Bob