The general idea is that you have one of the two routers (2951A & 2951B) as a gateway e.g 2951A. On your gateway router 2951A, create a PBR policy and apply it to its internal interface (the one facing to your LAN), so that it redirects the traffic you want (e.g VLAN 140 traffic) to the appropriate router (1841).
As traffic enters 2951A, it will match your VLAN 140 traffic via access lists and redirect it to the 1841 router.
The implementation is pretty straight forward.
If things are not that clear, post a diagram so we can be more specific.