Hi, what you are referring to is 1+1 HSB Line Protection Mode (vs. Split Protection Mode with optical splitter). There is more detail in the PTP820S Technical Description Document starting on page 52 of v10.0. The main caveat is that line protection mode is only certified on Cisco switches supporting LACP. Page 140 of the PTP820C&S User Guide v10.0 goes into more detail on the actual configuration.
I'm not sure about a redundant router setup, but I believe, from a PTP820 perspective, it is the same setup as two different network inputs into the radio. Essentially the PTP820 would have it's ETH1 and ETH2 ports in a LAG configuration with a PIPE service to the radio interface. The networking equipment would be responsible for deciding which (or both) ETH interfaces are sending data. Page 123 of the PTP820C&S User Guide v10.0 goes into more detail.
Most important for you would be following the instructions on page 140 of the PTP820C&S User Guide v10.0. LACP must be enabled on the router ports. Now, enable LACP on the Ethernet interface connected to the external router (i.e. ETH1) on the active PTP820:
i Select Ethernet > Interfaces > Logical Interfaces. The Logical Interfaces page opens .
ii Select the interface and click Edit. The Logical Interfaces – Edit page opens.
iii In the Interface Mode field, select LACP.
iv Click Apply, then Close.
In your case, repeat for ETH2. Once you copy to mate, the standby unit will have the same configuration.
Also, just for reference, the PTP820 has the ability to do automatic state propagation for controlling port status based on radio status (degredation, up/down, etc.). It is beyond what you are trying to do, but may be useful in the future. I've also attached a document that isn't applicable to your current design, but may be also useful in the future.
Ensure that you are running v10.5 of the firmware, and you may need to open a support case if you have the above setup correct and are still seeing significant delay in failover.
http://community.cambiumnetworks.com/t5/Licensed-Microwave/PTP820C-S-and-LACP/m-p/88614#M1025