Hi,
Have you tried adding a static NAT for the router's external interface which is located behind the inside interface of the ASA ?
Example .. let's say the router which is behind the inside (higher priority) interface of the ASA is 10.10.10.10 then you could add a static as below
static (inside,outside) 10.10.10.10 10.10.10.10 mask 255.255.255.255
Note: the above assumes that the second router is behind the outside interface (lower priority) of the ASA and that the second router knows how to reach 10.10.10.10. Obviously 10.10.10.10 should also know how to get to the second router.
next you will need to allow GRE on both interfaces.
access-list inside-out permit GRE host 10.10.10.10 host
access-list outside-in permit GRE host host 10.10.10.10
access-group inside-out in interface inside
access-group outside-in in interface outside
Give it a try ..
I hope it helps .. please rate it if it does !!!
