Restore from device and selecting the two above back ups
By doing that you're telling SQL that is is one backup striped across two files, which it is not.
Select restore, select just the full backup and make sure you find the option to leave the database in recovery (on the second tab of the restore backup screen)
Once that restore has completed, then go and restore again, this time selecting the diff backup. You can set the option to recover the DB and make it available on this won.
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: Discussions on DB performance with occasional diversions into recoverability
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