I hope I remember
my two subnets are 192.168.6.0/24 and 192.168.7.0/24
on the 192.168.7 router I have the rules
/ip firewall filter add action=accept chain=forward dst-address=192.168.7.0/24 src-address=192.168.6.0/24
/ip firewall filter add action=accept chain=input dst-address=192.168.7.0/24 src-address=192.168.6.0/24
on the 192.168.6 router I have the mirror rules
/ip firewall filter add action=accept chain=forward dst-address=192.168.6.0/24 src-address=192.168.7.0/24
/ip firewall filter add action=accept chain=input dst-address=192.168.6.0/24 src-address=192.168.7.0/24
Also don't forget the nat bypass as mentioned by ditonet
Looking at the chain it seems that the second firewall rule above does not match any packet so it may be useless
But now that it works, I am not really feeling like changing anything !
I hope it will help you ...