problem with spf from backup mx

Hi!
I don't know if this problem is for this forum, but maybe somebody can
help me.
I have two servers (primary and backup) with postfix and spf policy.
Problem is when my primary is off, backup get all mails.
After I turn on my primary, it gets mails from my backup server. But
reject them with spf policy that email and IP (number IP is my backup
server IP) are not match.
Is it possible to set-up backup server to forward mails to my primary
with correct IP (email domain IP)?

Regards,
Krystian