This guide assumes the following:

“ISP1” is your 1st wan connection name “ISP2” is your 2nd wan connection name “LAN” is your local network name

“ISP1” recieves the IP 111.111.111.1/24 on the network 111.111.111.0/24 “ISP2” recieves the IP 222.222.222.1/24 on the network 222.222.222.0/24

1. If your ISP assigned IP's via DHCP be sure to do the following (if they are assigned static move to step 2):

Log into the routeros webfig or winbox:

  IP
  DHCP Client
  Click on your first WAN DHCP client
  Change "add default gateway" to no
  Repeat the same for your second WAN connection

2. Remove current routing rules

  IP
  Firewall
  Nat
  Remove the entry for "masquerade" to your current single ISP.

3. SSH into your router and run the following script:

/ ip firewall mangle
add chain=prerouting dst-address=111.111.111.0/24  action=accept in-interface=LAN
add chain=prerouting dst-address=222.222.222.0/24  action=accept in-interface=LAN
add chain=prerouting in-interface=ISP1 connection-mark=no-mark action=mark-connection new-connection-mark=ISP1_conn
add chain=prerouting in-interface=ISP2 connection-mark=no-mark action=mark-connection new-connection-mark=ISP2_conn
add chain=prerouting  in-interface=LAN connection-mark=no-mark dst-address-type=!local per-connection-classifier=both-addresses:2/0 action=mark-connection new-connection-mark=ISP1_conn
add chain=prerouting  in-interface=LAN connection-mark=no-mark dst-address-type=!local per-connection-classifier=both-addresses:2/1 action=mark-connection new-connection-mark=ISP2_conn
add chain=prerouting connection-mark=ISP1_conn in-interface=LAN action=mark-routing new-routing-mark=to_ISP1
add chain=prerouting connection-mark=ISP2_conn in-interface=LAN action=mark-routing new-routing-mark=to_ISP2
add chain=output connection-mark=ISP1_conn action=mark-routing new-routing-mark=to_ISP1     
add chain=output connection-mark=ISP2_conn action=mark-routing new-routing-mark=to_ISP2
 
/ ip route
add dst-address=0.0.0.0/0 gateway=111.111.111.1 routing-mark=to_ISP1 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=222.222.222.1 routing-mark=to_ISP2 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=111.111.111.1 distance=1 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=222.222.222.1 distance=2 check-gateway=ping
 
/ ip firewall nat 
add chain=srcnat out-interface=ISP1 action=masquerade
add chain=srcnat out-interface=ISP2 action=masquerade