Make sure you use 'we' throughout the letter instead of 'I'.
There are two widely accepted options for the placement of 2 signatures. First, you may place the second signature below the first. You may also place the second signature using the same lines as the first but indented to the center of the page.
The first signature is always the more senior person.
For example:
or
____________ Signature
Name of the more senior person
____________ Signature
Name of the other person
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