You can not control the path the return traffic will take, but you can configure your router to policy route certain users out one serial link or another. For example, if you have a router with one ethernet and two serial links (assuming you have a class C network)
interface ethernet 0
ip policy route-map internet
!
route-map internet permit 10
match ip address 100
set interface s0
route-map internet permit 20
match ip address 101
set interface s1
!
access-list 100 permit 10.10.10.0 0.0.0.127
access-list 101 permit 10.10.10.128 0.0.0.127