@planetinse
Comparable to, for example, a Microsoft Windows back (Restore point ?), you can not backup a config on one device - let's call it pfSense1 and then restore it on another device, called pfSense2.
That is, this is possible if you are willing to edit the config file.
Only experts should do this, and everybody else willing to accept the consequences.
Consequences are :
The hardware NIC drivers names, like em0 or igb1 should be identical.
Be aware : the unique pfSense device ID should not the same :
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and true : the could be identical.
You could try this :
Copy the 'block' with HAProxy package settings from one config to the other.
Be aware (I never used HAProxy) : there could be several section.
Be aware : if, for example, HAProxy uses certificates from the certificate pfSense store, the certificate reference should be checked and probably for the destination pfSense.
Be aware : the will be most probably other things to check.
I guess you get it by now : it's an trail and error process, where you need to start worrying when things "seems to work" as they didn't fail yet ^^