All your going to need to do is Routing mark in Mangle and run static routes that refer to those routing marks. These can be one pool or two pools does not matter.
Assuming you have static IP’s via the DHCP client… Do not add default route to routing table for both clients
**If Ip’s are dynamic you will go ahead and add the routes but use a routing filter to differentiate them and add the routing marks
Add IP Pool for each local group
In PPP setup two profile one pulling from each IP Pool
In Firewall Mangle place routing marks on each IP range for each pool. The routing mark are what you will use to direct the traffic out different interfaces.
In routes add the two WAN routes and specify the routing marks used in mangle.
**If these were dynamic instead of static the route will already be in place and you will need to use the routing filter to add these routing marks.
Setup PPP Secret for each user applying the profile you want to each.
Try this and if you are having an issue please explain it clearly and post the /print of the rules being used.