There are many ways to skin the cat as mkx and rextended say.
So yes,
If you just have this,
/ip firewall mangle
add chain=prerouting src-address=192.168.X.X action=mark-routing new-routing-mark=WAN2
Any traffic from that LANIP should go out WAN2. That means ANY TRAFFIC!! Think about it.
Also, YES to your question
a. you can reach 192.168.X.X from a specific WAN ( but the associated return traffic has to go out the same WAN )
b. yes, you can go out with other traffic from 192.168.X.X on the other WAN.
But this is foolish speculation. YOu need to determine your actual requirements PRIOR to configuring the router.
Therefore, in detail
a. identify the user(s)/device(s) and groups of users/devices including you the admin
b. identify the traffic they should be able to accomplish.
Once we KNOW all the requirements, then a config can be achieved.
I am not going to be part of chasing you changing your mind every post...................... either you have a plan or you dont LOL.