You only place the IP PIN for the person who is listed first on the return when you are filing by mail.
Here are the instructions from Form 1040 for signature section and IP PIN -
"If you are filing a joint return and
both taxpayers receive an IP PIN, only
the taxpayer whose social security number
(SSN) appears first on the tax return
should enter his or her IP PIN. However,
if you are filing electronically, both taxpayers
must enter their IP PINs."
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf (page 76)
