You don't need to create new .p12 development and distribution certificates. You use those same certificates for all your apps. But for the second app, you need to create an new App ID and mobile provision files based on that App ID. If you're enabling push notifications, you also need different .p12 push certificates.
Use same for all apps:
.p12 development certificate
.p12 distribution certificate
Required for each app:
development mobile provision file
distribution mobile provision file
.p12 development push cert
.p12 distribution push cert