How to adapt the queue below to work with two network: 192.168.88.0/24 and 192.168.89/24?
I suppose you mean that you want to make this one marking rule apply the same mark to packets from both the 88 and 89 networks, so they'll have to share the same queue / policy elsewhere?
change 192.168.88.0/24 in the connection mark rule to be 192.168.88.0/23
(change the 24 to a 23)
change src-address=192.168.88.0/24 to be: src-address-list=LAN Networks
Then create the two networks as addresses in the address-list:
/ip firewall address-list
add list="LAN Networks" address=192.168.88.0/24
add list="LAN Networks" address=192.168.89.0/24
Any time you want to put more networks in the same policy - just add them to this list.
Furthermore, you may want to go to your filter and NAT rules and change src-address=192.168.88.0/24 to use the src-address-list just like in Mangle. Of course, this means that the two networks will get the same policy, etc. If you want them to have different filter rules, then you may not want to make the filter rules use the address list.