I have a case for a customer where dual serial interface are connected to same router at CE and PE end.
Requirement is to bundle both the Serial links using Multilink feature to be able to support active/active configuration for high availability.
Going forward we want to enable internet on the same last mile, as I understand using same last mile for both internet and MPLS will require two things on the router:
1) Creation of Subinterface on serial interface (Requires the encapsulation to be changed to Frame Relay)
2) NAT support on the Subinterface for internet traffic
To summarize the requirement we need Multilink feature with option of running internet and MPLS service on the Multilink group using subinterface and NAT configuration.