I'm not sure what exactly you want to do, but basically if you set the specific route for both devices, any IP/Host that is not directly connected, what means is not on the addres table, the router will try to reach using the other router.
== R1 IP Address ==
== R2 IP Address =
- If R1 try to look for IP 192.168.15.250 (192.168.15.0/24 network) it wont look for the default route.
- If R1 look for 192.168.20.56 (192.168.20.0/24 network) it will look for the default route gateway and everything will work fine
- If R1 look for 192.168.12.22 (192.168.12.0/24 network) it will look for the default route gateway even the R2 router has no such IP on his route table. It will cause a routing loop, since none of the devices have such IP
OBS: In addition, you had to be carefull to never use the same networks on both routers.
After all, if you want to both sides see each other, you could change it all on one single network or a layer-2 network. I don't advice you to have those default routes