I don't know how much you know about multi-WAN config in general, and I don't want to explain everything from the beginning, that was done already many times by many people. But in short, very basic dual-WAN config can be:
/ip address
add address=111.167.10.1/?? interface=ether1
add address=192.168.15.1/?? interface=ether2
add address=192.168.1.1/24 interface=ether5
/ip route
add dst-address=0.0.0.0/0 gateway=111.167.10.?
add dst-address=0.0.0.0/0 gateway=192.168.15.? distance=2
add dst-address=0.0.0.0/0 gateway=111.167.10.? table=wan1
add dst-address=0.0.0.0/0 gateway=192.168.15.? table=wan2
Everything from LAN will use WAN1 by default if it's available. If not, it will switch to WAN2. It's the first two routes. Now if you want to force something to use specific WAN, you can do things like:
/ip firewall mangle
add chain=prerouting src-address=192.168.1.10 action=mark-routing new-routing-mark=wan2
add chain=prerouting src-address=192.168.1.20 protocol=tcp dst-port=80,443 action=mark-routing new-routing-mark=wan2
It will force everything from 192.168.1.10 and web browsing from 192.168.1.20 to use WAN2.
You can find better config at
https://wiki.mikrotik.com/wiki/Manual:PCC. Even though it's about load balancing, it shows and explains general principles. I suggest to read and understand it. The actual load balancing is just the two rules with per-connection-classifier option.
And finally the main topic. If you add second address to be used as gateway for WAN2:
/ip address
add address=192.168.1.2/24 interface=ether5
it won't help you, because you can't tell which one client used. Router doesn't see that, because how it works is that clients uses ARP to resolve IP address to MAC address and it sends packets there. Destination IP address belongs to target server and IP address of gateway isn't contained anywhere.
The trick is to (mis)use VRRP (for the lack of better options):
/interface vrrp
add interface=ether5 name=vrrp1
/ip address
add address=192.168.1.2/32 interface=vrrp1 comment="mask /32 is important"
/ip firewall mangle
add chain=prerouting in-interface=vrrp1 action=mark-routing new-routing-mark=wan2
This way you get another MAC address on VRRP interface, so 192.168.1.1 and 192.168.1.2 will each have different one, and router will be able to tell which one client used. And that's it.