How to combine p12 certificate files

If you have 2 .p12 files (say certificates_1.p12 and certificates_2.p12 ), each encrypted with its own password, you can merge certificates_1.p12 into certificates_2.p12 with the following command:

keytool -importkeystore -srckeystore certificates_1.p12 -srcstoretype PKCS12 -destkeystore certificates_2.p12 -deststoretype PKCS12

This was the case when we wanted to update the certificates we use to code-sign our macOS app and we needed to combine the old certificates with the new ones so that the automatic update system would work.