If you want to sign an APK more than once, just do so.
But note that Google Play does not accept APKs with multiple signatures.
eg. You can sign an APK from the command line using jarsigner like so:
jarsigner -keystore original-keystore my-app-unsigned.apk key-alias alias
Then just repeat this with the second key:
jarsigner -keystore new-signing-keystore my-app-unsigned.apk key-alias
Don’t forget to run zipalign afterwards:
zipalign -v 4 my-app-unsigned.apk my-app.apk
I just re-read the part about MotoDev studio. In that case you can just sign the APK first using MotoDev as usual, then sign with your new key on the command line as above.