[mcnallym]

This isn't how ISP Redundancy is designed.

It is designed so that either use both lines in Load Sharing where you implement your own DNS Server internally for your public and the load sharing is done by the DNS Proxy alternating which ISP address it gives out, or in Active BAckup where sends all traffic down one line until it fails and then swaps over.

If the remote src/dst is known then you can implement static routes to send the traffic down the alternative line however you can't do service based routing, and this is purely normal address based routing.