Оказалось всё не просто, а очень просто.
Создал GRE тунели по числу IP каналов, включил per-packet load-balancing и CEF сделал всё, что требовалось.ip cef
interface Tunnel0
ip address 192.168.88.34 255.255.255.252
ip load-sharing per-packet
tunnel source ...
tunnel destination ...
interface Tunnel1
ip address 192.168.88.38 255.255.255.252
ip load-sharing per-packet
tunnel source ...
tunnel destination ...
ip route 192.168.1.0 255.255.255.0 192.168.88.33
ip route 192.168.1.0 255.255.255.0 192.168.88.37
На второй - симметрично.
ip cef
interface Tunnel0
ip address 192.168.88.33 255.255.255.252
ip load-sharing per-packet
tunnel source ...
tunnel destination ...
interface Tunnel1
ip address 192.168.88.37 255.255.255.252
ip load-sharing per-packet
tunnel source ...
tunnel destination ...
ip route 192.168.1.0 255.255.255.0 192.168.88.34
ip route 192.168.1.0 255.255.255.0 192.168.88.38